Fix a fuzzer crash in VideoFramePool

It seems that providing random data to the VPX parser can make it call the video
frame pool with illegal parameters. This currently causes a crash, let's make it
return null, and the calling code already seems to handle that well.

BUG=648849

Review-Url: https://codereview.chromium.org/2360373002
Cr-Commit-Position: refs/heads/master@{#420443}
1 file changed