This patch fix large log output and slowdown for captured video
caused by calling CMSampleBufferGetImageBuffer with no video frame.
This could be fixed by bumping ffmpeg to
02f65678ba9b5958cf53d1e5bc29939d941ad95f, but it cause other
problems(crash when switching to screen sharing)

Change-Id: I0dc33683f1e40ce151130774316892f7f4730f83