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()`.
