[WebRTC] Add missing assertions to rtp_format_h264.cc/rtp_format_h265.cc.
<rdar://problem/118259276>
Pull request: https://github.com/WebKit/WebKit/pull/20352
Committed 270677@main (746712ecb50e): <https://commits.webkit.org/270677@main> Reviewed commits have been landed. Closing PR #20352 and removing active labels.