screencast: Discard pending buffer and fence if stream state changes

While the changes made earlier prevent enqueueing buffers when the
stream is not in a streaming state, screencasting continued to crash.
The reason for that seems to be that pipewire will, in certain cases,
invalidate some buffers, including the buffer that we are currently
tracking as "pending buffer". The result of this is that when we then
try to enqueue that buffer, we get a crash.

To prevent this, discard the pending buffer when the stream state
changes. Since that makes the pending fence useless as well, also
discard it and its notifier.


(cherry picked from commit 8d0e1b29)