[isolate] Add null checks to native_context getters

Having had these would have saved me quite a bit of time :)

Change-Id: I36a04cc89db9ceaa786a146d01a6b0f4444f5729
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1914214Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65015}

src/execution/isolate-inl.h

@@ -26,10 +26,12 @@ void Isolate::set_context(Context context) {
 }
 
 Handle<NativeContext> Isolate::native_context() {
+  DCHECK(!context().is_null());
   return handle(context().native_context(), this);
 }
 
 NativeContext Isolate::raw_native_context() {
+  DCHECK(!context().is_null());
   return context().native_context();
 }