GetInputPin() and GetOutputPin() do not guarantee returning non-null pins.
Should either of them return null during Init() we're better off returning an
error than allowing unsafe behavior further ahead.
Bug: webrtc:9941
Change-Id: I25858f0555334b4ef99801f83454931384695bf6
Reviewed-on: https://webrtc-review.googlesource.com/c/108603
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Per Kjellander <perkj@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25475}