When GetSources is called with non-existing ssrc, it will log the
error and return an empty RtpSource list instead of hitting the DCHECK.
Bug: chromium:793699
Change-Id: I30bebb657de32f87f9c82920fa0b19403893791f
Reviewed-on: https://webrtc-review.googlesource.com/32860
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Zhi Huang <zhihuang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21258}
fa266efb27