oggdemux: Solidify gst_ogg_demux_loop_push() some more

oggdemux: Solidify gst_ogg_demux_loop_push() some more

There were still some races going on where seeking events wouldn't
be properly intercepted/executed by this thread.

* Instead of always waiting for the GCond to be emitted, first just
  check if there is an event available
* Take ownership of the event *while* the lock is taken and not
  after releasing/reacquiring it
* Finally acquire lock at the very top and release it at the end
  to make it a bit more streamlined

This removes the remaining issues with seeks not being executed