null-check the redirect context before using. (#65910)

* null-check the redirect context before using.

* tweak the comment

* do not allocate context if InitializeContext has unexpected results.

Author: VSadov

src/coreclr/vm/threadsuspend.cpp

@@ -1983,9 +1983,14 @@ CONTEXT* AllocateOSContextHelper(BYTE** contextBuffer)
         pfnInitializeContext2(NULL, context, NULL, &contextSize, xStateCompactionMask) :
         InitializeContext(NULL, context, NULL, &contextSize);
 
-    // The following assert is valid, but gets triggered in some Win7 runs with no impact on functionality.
-    // commenting this out to reduce noise, as long as Win7 is supported.
-    // _ASSERTE(!success && GetLastError() == ERROR_INSUFFICIENT_BUFFER);
+    // Spec mentions that we may get a different error (it was observed on Windows7).
+    // In such case the contextSize is undefined.
+    if (success || GetLastError() != ERROR_INSUFFICIENT_BUFFER)
+    {
+        STRESS_LOG2(LF_SYNC, LL_INFO1000, "AllocateOSContextHelper: Unexpected result from InitializeContext (success: %d, error: %d).\n",
+            success, GetLastError());
+        return NULL;
+    }
 
     // So now allocate a buffer of that size and call InitializeContext again
     BYTE* buffer = new (nothrow)BYTE[contextSize];
@@ -2896,9 +2901,15 @@ BOOL Thread::RedirectThreadAtHandledJITCase(PFN_REDIRECTTARGET pTgt)
     if (!pCtx)
     {
         pCtx = m_pSavedRedirectContext = ThreadStore::GrabOSContext(&m_pOSContextBuffer);
-        _ASSERTE(GetSavedRedirectContext() != NULL);
     }
 
+    // We may not have a preallocated context. Could be short on memory when we tried to preallocate.
+    // We cannot allocate here since we have a thread stopped in a random place, possibly holding locks
+    // that we would need while allocating.
+    // Other ways and attempts at suspending may yet succeed, but this redirection cannot continue.
+    if (!pCtx)
+        return (FALSE);
+
     //////////////////////////////////////
     // Get and save the thread's context
     BOOL bRes = true;