gstrtpjitterbuffer can deadlock

bf00ee46 introduced a deadlock. Basically, it is possible for gst_rtp_jitter_buffer_loop to end up in JBUF_WAIT_EVENT() inside the loop, while the _chain() function ends up in JBUF_WAIT_QUEUE(), and we are waiting forever.

I have yet to try and write a test for this, but architecturally I hope we could avoid having two threads waiting on each-other like that. (and a test to go with such a change to document what it is trying to do would not be a bad idea either) :)