roll: pigweed pw_bluetooth_proxy: Fix undefined behavior in DeregisterChannelLocked

Fix dereference of the channels_.end() iterator, which UBSan flagged as
undefined behavior in a downstream project.

This is already covered by tests like
ChannelProxyTest.ChannelsStopOnProxyDestruction, but the UBSan failure
is not triggered until member variables are added to L2capChannelManager
that move the location of channels_ out of 16-byte alignment. The end()
iterator is a list item base class without a L2capChannel subclass, so
dereferencing it tries to downcast to a non-existent subclass.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/331313
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: 6b148032eba5cfc22d33a5a953401dbfc276578c

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: d390baa10ed74e..6b148032eba5cf
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8701330397668287889
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib13ea091faa33be07dc10a5b8c2bdcb91e681f6a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/showcase/sense/+/331433
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>