This fixes a random crash with stop-and-forget capture.
When aborting capture, the disk-writer can emit
midi_write_source->drop_references ()
in the butler thread, which leads to a direct call to
Session::remove_source.
This can happen before or after Region::source_deleted
is called which is initiated from the same signal.
Furthermore it was possible that Region::source deleted
was called concurrently from the UI thread via SourceRemoved
for whole file regions, which lead to memory corruption.
ad49de022a