Make sure nullptr is returned if the packet is not in history.
Bug: webrtc:9863
Change-Id: I9658b1b271071a4bd38f062ed68c60cc04c63123
Reviewed-on: https://webrtc-review.googlesource.com/c/105300
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25114}