Build error with GCC13.2 and -flto

[cpackham-atlnz]

armv7,apteryx: lto-wrapper: warning: using serial compilation of 5 LTRANS jobs
armv7,apteryx: lto-wrapper: note: see the '-flto' option documentation for more information
armv7,dbus: checking for clearenv... yes
armv7,dbus: checking for unsetenv... yes
armv7,apteryx: callbacks.c: In function 'test_cb_release':
armv7,apteryx: callbacks.c:914:5: error: '__atomic_load_4' writing 4 bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=]
armv7,apteryx:   914 |     CU_ASSERT (g_atomic_int_get (&cb->refcnt) == 1);
armv7,apteryx:       |     ^
armv7,apteryx: lto1: note: destination object is likely at address zero

haven't dug into it yet.

[cpackham-atlnz]

Could be a false positive. cb_create does g_atomic_int_set (&cb->refcnt, 1); then g_atomic_int_inc (&cb->refcnt); so you'd expect refcnt to be 2 upon return (which is a bit strange but whatever). So the test code that does the first cb_release() should leave us with a valid object with a refcnt of 1.