pw_bluetooth_sapphire: Fix L2CAP Signaling Channel UAF

Fixes a Use-After-Free (UAF) vulnerability in the BR/EDR L2CAP signaling
channel. Multiple commands can be sent in a single C-frame, which are
parsed and dispatched in a loop within
`BrEdrSignalingChannel::DecodeRxUnit`. If a dispatched command causes
a link error, the `LogicalLink` and consequently the
`BrEdrSignalingChannel` are synchronously destroyed. We prevent UAF by
checking if the signaling channel is still alive using a weak pointer
after each command dispatch, and returning early if it was destroyed.

Fixed: b/510065914
Fixed: b/535160279
Change-Id: Ib8c72dcb0acbfb6036aa6671cbb3b61a0a5e97f6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/482627