Ensure m_backend is valid during use in RTCRtpTransform::attachToReceiver
<rdar://problem/96589679>
Pull request: https://github.com/WebKit/WebKit/pull/2166
Committed 252232@main (f9969ede8c64): <https://commits.webkit.org/252232@main> Reviewed commits have been landed. Closing PR #2166 and removing active labels.