Check for non-existent context before unwrapping debug evaluate context

This CL attempts to fix a chrome crash seen in the wild. Without a
reproducer, the current working theory is that we hit a 'null' context
in some edge case, causing us to access an empty handle. This CL
prevents the empty context handle to be dereferenced.

TBR=yangguo@chromium.org

Bug: chromium:1038747
Change-Id: Icd6f4853a22ddbf1e504f0f0f90c065b3437f8ab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000752Reviewed-by: Simon Zünd <szuend@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65776}

src/debug/debug-scopes.cc

@@ -300,7 +300,7 @@ void ScopeIterator::TryParseAndRetrieveScopes(ReparseStrategy strategy) {
 }
 
 void ScopeIterator::UnwrapEvaluationContext() {
-  if (!context_->IsDebugEvaluateContext()) return;
+  if (context_->is_null() || !context_->IsDebugEvaluateContext()) return;
   Context current = *context_;
   do {
     Object wrapped = current.get(Context::WRAPPED_CONTEXT_INDEX);