Don't crash when an image fails to decode.

The code that extracts the image for the default frame from an animated
bitmap should check that the image data is actually available.
It is possible for decoding to fail for invalid frames, Blink should not
crash when this happens.

BUG=408026

Review URL: https://codereview.chromium.org/535823002

git-svn-id: svn://svn.chromium.org/blink/trunk@182496 bbb929c8-8fbe-4397-9dbb-9b2b20218538