Fix async stack walk crash for continuations with null DiagnosticIP (#128496)

CordbAsyncStackWalk::PopulateFrame() crashes when encountering a
continuation whose ResumeInfo.DiagnosticIP is NULL (e.g. the new
ValueTaskContinuation introduced in #127973). GetNativeCodeInfoForAddr
is called with a null address which fails.

Fix: In PopulateFrame(), skip continuations with diagnosticIP == NULL
the same way DiagnosticHidden frames are skipped (advance to Next).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: tommcdon

src/coreclr/debug/di/rsstackwalk.cpp

@@ -915,6 +915,15 @@ HRESULT CordbAsyncStackWalk::PopulateFrame()
             &nextContinuation,
             &state));
 
+        // Skip continuations with null DiagnosticIP. These are infrastructure
+        // continuations (e.g. ValueTaskContinuation) that have no user code
+        // associated with them and cannot be represented as a debug frame.
+        if (diagnosticIP == 0)
+        {
+            m_continuationAddress = nextContinuation;
+            continue;
+        }
+
         NativeCodeFunctionData codeData;
         VMPTR_Module pModule;
         mdMethodDef methodDef;