[vm] Fix error notification when unable to print the stacktrace.

TEST=run test harness with low heap limit
Change-Id: I8084c50c3aab1c0cbfa32bbcc5df100abe4211bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/213537
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/isolate.cc

@@ -2315,8 +2315,12 @@ bool Isolate::NotifyErrorListeners(const char* message,
   msg.value.as_string = const_cast<char*>(message);
   arr_values[0] = &msg;
   Dart_CObject stack;
-  stack.type = Dart_CObject_kString;
-  stack.value.as_string = const_cast<char*>(stacktrace);
+  if (stacktrace == NULL) {
+    stack.type = Dart_CObject_kNull;
+  } else {
+    stack.type = Dart_CObject_kString;
+    stack.value.as_string = const_cast<char*>(stacktrace);
+  }
   arr_values[1] = &stack;
 
   SendPort& listener = SendPort::Handle(current_zone());

runtime/vm/message_snapshot.cc

@@ -3162,6 +3162,7 @@ bool ApiMessageSerializer::Trace(Dart_CObject* object) {
       cid = kDoubleCid;
       break;
     case Dart_CObject_kString: {
+      RELEASE_ASSERT(object->value.as_string != NULL);
       const uint8_t* utf8_str =
           reinterpret_cast<const uint8_t*>(object->value.as_string);
       intptr_t utf8_len = strlen(object->value.as_string);