Check for NULL dib in RenderWidgetHostImpl::OnShowDisambiguationPopup().

This is a quick browser crash when the IPC fuzzer sends a bad mesage to
the browser, and is easy to handle properly.

BUG=336622

Review URL: https://codereview.chromium.org/132693023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246496 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246496