Fix potential nullptr dereference

We found this while trying to integrate V8 6.3 into Node.js. One of the
tests started to crash on Windows.

https: //github.com/nodejs/node/pull/16271#issuecomment-337790715
Bug: 
Change-Id: I82514ff7b9ca6a2b5c4489fe7388c4beda9931c9
Reviewed-on: https://chromium-review.googlesource.com/735400Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Cr-Commit-Position: refs/heads/master@{#48899}

src/frames.cc

@@ -1892,10 +1892,10 @@ void JavaScriptFrame::Print(StringStream* accumulator,
   Context* context = nullptr;
   if (this->context() != nullptr && this->context()->IsContext()) {
     context = Context::cast(this->context());
-  }
-  while (context->IsWithContext()) {
-    context = context->previous();
-    DCHECK_NOT_NULL(context);
+    while (context->IsWithContext()) {
+      context = context->previous();
+      DCHECK_NOT_NULL(context);
+    }
   }
 
   // Print heap-allocated local variables.