pc: fix use-after-free in AllocateSctpSids

AllocateSctpSids iterates over newly assigned channels using raw
pointers and calls OnTransportReady on each one. OnTransportReady
can synchronously fail (e.g. if sending the DCEP OPEN message
fails), which may cause the channel to close and be deleted,
leaving the controller with a dangling pointer.

This change ensures the channels are kept alive during the loop by
using scoped_refptr for the temporary collection. This fixes the
UAF for any failure mode that prevents synchronous sending of the
initialization message.

Bug: chromium:503422316
Change-Id: I1b927386fb4ed4279036835a244320aba6acf875
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/465701
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47503}