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