decodebin3: fix collection leaks

- gst_message_parse_stream_collection() returns an owned collection.
- get_merged_collection() returns one as well and was leaked in the
  else block.

Fix leak when running:
GST_TRACERS=leaks GST_DEBUG="GST_TRACER:7,leaks:6" gst-play-1.0 --use-playbin3 test.mkv

Part-of: <gstreamer/gstreamer!954>
24 jobs for !954 with gst-plugins-base-playbin-leak in 52 minutes and 9 seconds (queued for 1 second)
merge request