gst/gstbus.c: Only allow one bus watch to be set at a time. This is necessary because...

gst/gstbus.c: Only allow one bus watch to be set at a time. This is necessary because the dispatcher pops the message...

Original commit message from CVS:
Base on Patch by: Olivier Crete <tester at tester dot ca>
* gst/gstbus.c: (gst_bus_source_finalize),
(gst_bus_add_watch_full), (gst_bus_add_signal_watch_full):
Only allow one bus watch to be set at a time. This is necessary
because the dispatcher pops the message from the bus and the second
watcher will then get NULL or the next message (and the first won't
get this next message then, etc). If more than one "watcher" is
required signal watches should be used. Fixes bug #526044.