Previously, a client disconnecting while a sample was playing could
lead to issues. For example, if a client disconnected before the
"ready" signal of the sample-play arrives, `operation_new_cb()`
would be called and that would try to use the client's pw_manager,
however, that has previously been destroyed in `client_disconnect()`.
If the client disconnected after the "ready" signal but before the reply
has been sent, then `sample_play_ready_reply()` would never be called
since operations are completed via the client's pw_manager which
would already be destroyed at that point.
Fix this by installing a listener on the client, and properly
cancelling the operation and making sure that the pending_sample
is correctly destroyed.
If the sample finished playing before we finished the roundtrip to
get the sink_index, it will be destroyed. When the roundtrip completes,
it will try to use invalid memoryy and crash.
Make sure we destroy all pending replies before destroying the sample
to avoid this problem.
Fixes#2151
Move the reference counting from `pending_sample_free()`
into `on_sample_done()` so that the client's references
are managed in a single place.
The reason why `pending_sample_free()` cannot simply call
`client_unref()` is that `client_free()` may be called from
`manager_disconnect()` regardless of the reference count,
and, in turn, `pending_sample_free()` may be called,
which could then lead to a recursive call to `client_free()`.
