Fix mirroring shutdown crashes.

These client_ members are not owned, so they may be destructed during
the shutdown process. To avoid segfaults, we should set the clients to
null after informing them that the session has ended. We should also
check that the client is not null before accessing it.

Test:
Cast to 3P linux cast receiver. In tandem with crrev.com/c/6533602 (in
the third_party/openscreen/src repo), I no longer see crashes when
stopping mirroring (via data timeout or the user manually stopping the
session).

Bug: b:374199735
Change-Id: Iaa3549a861222977a97ebf41fdd7e9d3504f19e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6579762
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Antonio Rivera <antoniori@google.com>
Cr-Commit-Position: refs/heads/main@{#1466672}