inspector: speculative crash fix

We might have empty frame_inspector_ here, let's fast return in this
case.

R=yangguo@chromium.org

Bug: chromium:864920
Change-Id: Ia8012251877bb4e04b7f756db72a3039bca6fa9c
Reviewed-on: https://chromium-review.googlesource.com/1250408Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56278}

src/debug/debug-stack-trace-iterator.cc

@@ -117,7 +117,9 @@ v8::MaybeLocal<v8::Value> DebugStackTraceIterator::GetReceiver() const {
 
 v8::Local<v8::Value> DebugStackTraceIterator::GetReturnValue() const {
   DCHECK(!Done());
-  if (frame_inspector_->IsWasm()) return v8::Local<v8::Value>();
+  if (frame_inspector_ && frame_inspector_->IsWasm()) {
+    return v8::Local<v8::Value>();
+  }
   bool is_optimized = iterator_.frame()->is_optimized();
   if (is_optimized || !is_top_frame_ ||
       !isolate_->debug()->IsBreakAtReturn(iterator_.javascript_frame())) {