Make sure that ffmpeg returning us an invalid stream cannot damage us too

      badly.

BUG=62127
TEST=Added by Andrew

Review URL: http://codereview.chromium.org/4619001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@65346 0039d316-1c4b-4281-b951-d872f2087c98