This causes a crash if the NV12 texture cache attempts to upload textures
for a frame with a NULL backing CVPixelBufferRef.
Bug: webrtc:10175
Change-Id: I6866dcde5ace745cbd95b762254294aa8406c2a5
Reviewed-on: https://webrtc-review.googlesource.com/c/115430
Commit-Queue: Chuck Hays <haysc@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26140}