Original commit message from CVS:
* gst/tcp/gstmultifdsink.c: (gst_multi_fd_sink_add_full),
(gst_multi_fd_sink_remove), (gst_multi_fd_sink_clear),
(gst_multi_fd_sink_get_stats),
(gst_multi_fd_sink_remove_client_link),
(gst_multi_fd_sink_queue_buffer),
(gst_multi_fd_sink_handle_clients):
* gst/tcp/gstmultifdsink.h:
Make using the remove or clear signals threadsafe.
Make calling get-stats with an invalid fd not segfault.
Fixes 368273.