gst/gstbus.c: Fix deadlock, g_source_get_id() cannot be called in finalize.

gst/gstbus.c: Fix deadlock, g_source_get_id() cannot be called in finalize.

Original commit message from CVS:
* gst/gstbus.c: (gst_bus_source_finalize),
(gst_bus_add_watch_full_unlocked), (gst_bus_add_watch_full),
(gst_bus_enable_sync_message_emission),
(gst_bus_disable_sync_message_emission),
(gst_bus_add_signal_watch_full), (gst_bus_remove_signal_watch):
Fix deadlock, g_source_get_id() cannot be called in finalize.
Keep track of the watch source by keeping a pointer to the source object
instead.
Use the bus lock to protect access to the pointer to the current
watch source.