Put the null pointer check for theme images before we use the pointer.
Put the null pointer check for theme images before we use the
pointer.

This was crashing for me with the theme I had installed.

Review URL: http://codereview.chromium.org/160550


git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@22342 0039d316-1c4b-4281-b951-d872f2087c98

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