[restapi] Fix nil pointer dereference in monitor_certs when cert file is missing (#185)

#### Why I did it

`monitor_certs()` in `go-server-server/main.go` calls `os.Lstat()` on each cert file path but silently discards the error return value with `_`. When a cert file is missing or its path is invalid, `os.Lstat()` returns `(nil, error)`. Calling `.ModTime()` on the nil `FileInfo` immediately causes a **nil pointer dereference (SIGSEGV)** that crashes the entire restapi process.

Observed crash in SONiC nightly CI (Arista 4600C, image 20251110.22):
```
goroutine 1 [running]:
main.monitor_certs(0xc000056060, 0xc000012100)
 go-server-server/main.go:122 +0x...
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation addr=0x20 pc=...]
```

After the crash, `supervisord` restarted restapi and the proc-exit-listener emitted ERR alerts every 60 seconds, causing `system_health/test_system_status` to fail.

##### Work item tracking
- Microsoft ADO: [37693847](https://msazure.visualstudio.com/One/_workitems/edit/37693847)

#### How I did it

Added explicit nil/error checks after every `os.Lstat()` call in `monitor_certs` (6 total):

- **Startup section (3 calls):** use `log.Fatalf` so the process fails immediately with a descriptive error message instead of panicking with an opaque SIGSEGV.
- **Monitor loop (3 calls):** use `log.Printf` + `time.Sleep` + `continue` so a transiently missing cert file is retried on the next cycle (every 3600 s) without crashing the server.

#### How to verify it

1. Start restapi with `--https` flag and a cert path that does not exist on disk.
2. **Before fix:** process crashes with `panic: runtime error: invalid memory address or nil pointer dereference`.
3. **After fix:** process exits cleanly via `log.Fatalf` with `"error: couldn't stat client cert file: ..."`.

#### Which release branch to backport (provide reason below if selected)
- [ ] 202305
- [ ] 202311
- [ ] 202405
- [ ] 202411
- [ ] 202505
- [x] 202511 — same crash observed on image 20251110.22 (202511 branch)

#### Tested branch (Please provide the tested image version)
- [x] 20251110.22 (202511) — crash confirmed in nightly CI job [69e79fb88e43924279229609](https://elastictest.org/scheduler/testplan/69e79fb88e43924279229609)

#### Description for the changelog

Fix nil pointer dereference crash in `monitor_certs` when a TLS cert file path is invalid or missing; restapi now exits with a descriptive error at startup instead of panicking silently.

Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>

Author: mssonicbld

go-server-server/main.go

@@ -102,23 +102,37 @@ func signal_handler(messenger chan<- int, wgroup *sync.WaitGroup) {
 
 func monitor_certs(messenger chan<- int, wgroup *sync.WaitGroup) {
     defer wgroup.Done()
-    client_cert_finfo, _ := os.Lstat(*sw.ClientCertFlag)
+    client_cert_finfo, err := os.Lstat(*sw.ClientCertFlag)
+    if err != nil {
+        log.Fatalf("error: couldn't stat client cert file: %s", err)
+    }
     prev_client_cert_mtime := client_cert_finfo.ModTime()
     log.Printf("trace: Last modified time of %s is %d", client_cert_finfo.Name(), prev_client_cert_mtime.Unix())
 
-    server_cert_finfo, _ := os.Lstat(*sw.ServerCertFlag)
+    server_cert_finfo, err := os.Lstat(*sw.ServerCertFlag)
+    if err != nil {
+        log.Fatalf("error: couldn't stat server cert file: %s", err)
+    }
     prev_server_cert_mtime := server_cert_finfo.ModTime()
     log.Printf("trace: Last modified time of %s is %d", server_cert_finfo.Name(), prev_server_cert_mtime.Unix())
 
-    sever_key_finfo, _ := os.Lstat(*sw.ServerKeyFlag)
+    sever_key_finfo, err := os.Lstat(*sw.ServerKeyFlag)
+    if err != nil {
+        log.Fatalf("error: couldn't stat server key file: %s", err)
+    }
     prev_sever_key_mtime := sever_key_finfo.ModTime()
     log.Printf("trace: Last modified time of %s is %d", sever_key_finfo.Name(), prev_sever_key_mtime.Unix())
 
     time.Sleep(CERT_MONITOR_FREQUENCY)
 
     for {
         reload := false
-        client_cert_finfo, _ := os.Lstat(*sw.ClientCertFlag)
+        client_cert_finfo, err := os.Lstat(*sw.ClientCertFlag)
+        if err != nil {
+            log.Printf("error: couldn't stat client cert file: %s", err)
+            time.Sleep(CERT_MONITOR_FREQUENCY)
+            continue
+        }
         client_cert_mtime := client_cert_finfo.ModTime()
         log.Printf("trace: Last modified time of %s is %d", client_cert_finfo.Name(), client_cert_mtime.Unix())
         if client_cert_mtime != prev_client_cert_mtime {
@@ -127,7 +141,12 @@ func monitor_certs(messenger chan<- int, wgroup *sync.WaitGroup) {
         }
         prev_client_cert_mtime = client_cert_mtime
 
-        server_cert_finfo, _ := os.Lstat(*sw.ServerCertFlag)
+        server_cert_finfo, err := os.Lstat(*sw.ServerCertFlag)
+        if err != nil {
+            log.Printf("error: couldn't stat server cert file: %s", err)
+            time.Sleep(CERT_MONITOR_FREQUENCY)
+            continue
+        }
         server_cert_mtime := server_cert_finfo.ModTime()
         log.Printf("trace: Last modified time of %s is %d", server_cert_finfo.Name(), server_cert_mtime.Unix())
         if server_cert_mtime != prev_server_cert_mtime {
@@ -136,7 +155,12 @@ func monitor_certs(messenger chan<- int, wgroup *sync.WaitGroup) {
         }
         prev_server_cert_mtime = server_cert_mtime
 
-        sever_key_finfo, _ := os.Lstat(*sw.ServerKeyFlag)
+        sever_key_finfo, err := os.Lstat(*sw.ServerKeyFlag)
+        if err != nil {
+            log.Printf("error: couldn't stat server key file: %s", err)
+            time.Sleep(CERT_MONITOR_FREQUENCY)
+            continue
+        }
         sever_key_mtime := sever_key_finfo.ModTime()
         log.Printf("trace: Last modified time of %s is %d", sever_key_finfo.Name(), sever_key_mtime.Unix())
         if sever_key_mtime != prev_sever_key_mtime {