loopback: Fix crash bug

loopback: Fix crash bug

The loopback message may be called after the sink input is already destroyed which causes
a crash. Also memory is leaked because the message object is not correctly freed.

This patch fixes the problems by adding a "dead" flag to the message structure and freeing
the message object on exit.

[jcsing.lee: backport mainline commit 468e3669a for fixing crash]

[Version] 13.0-31
[Issue Type] Bug fix

Change-Id: Id5ce55554fadda9cffeca698685897a88f45c2cb
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/541>