rtp-discoverer: Fix reference counting
rtp-discoverer: Fix reference counting

gst_caps_merge takes ownership of both arguments, fix the reference
counting to take that into account

kakaroto/farstream,pexip/farstream,tieto/farstream,tieto/farstream,kakaroto/farstream,pexip/farstream,pexip/farstream,tieto/farstream,kakaroto/farstream,tieto/farstream,kakaroto/farstream,tieto/farstream,pexip/farstream