Add error handling for SetBitrate when PeerConnection is closed

When SetBitrate is called after the PeerConnection has been closed,
it could lead to RTC_DCHECK (on debug builds) or null pointer dereferenceon release builds. This change adds appropriate error handling and a test case for this scenario.

Bug: webrtc:439455390
Change-Id: I1e51fe3c0477877b0239d5b378aa94c3247b8016
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/404980
Auto-Submit: Daniel.L (Byoungchan) Lee <daniel.l@hpcnt.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45379}