Don't crash in the case of an unexpected IPC response.

BUG=746967

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: Id9106a040641386eea32988ce44cd2324e36798b
Reviewed-on: https://chromium-review.googlesource.com/684657Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504482}

content/browser/frame_host/render_frame_host_impl.cc

@@ -2108,6 +2108,11 @@ void RenderFrameHostImpl::OnTextSurroundingSelectionResponse(
     const base::string16& content,
     uint32_t start_offset,
     uint32_t end_offset) {
+  // text_surrounding_selection_callback_ should not be null, but don't trust
+  // the renderer.
+  if (text_surrounding_selection_callback_.is_null())
+    return;
+
   // Just Run the callback instead of propagating further.
   text_surrounding_selection_callback_.Run(content, start_offset, end_offset);
   // Reset the callback for enabling early exit from future request.