ObjC: Add null checks to HW encoder compressionOutputCallback

This will help debugging.

BUG=b/65254613

Review-Url: https://codereview.webrtc.org/3012693002
Cr-Original-Commit-Position: refs/heads/master@{#19640}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 85d18d43adc679d8d3160106a918fe05fdbc0755
1 file changed