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>

subprojects/gst-plugins-base/gst/playback/gstdecodebin3.c

@@ -930,8 +930,11 @@ gst_decodebin3_input_pad_unlink (GstPad * pad, GstObject * parent)
       SELECTION_UNLOCK (dbin);
       gst_element_post_message (GST_ELEMENT_CAST (dbin), msg);
       update_requested_selection (dbin);
-    } else
+    } else {
+      if (collection)
+        gst_object_unref (collection);
       SELECTION_UNLOCK (dbin);
+    }
 
     gst_bin_remove (GST_BIN (dbin), input->parsebin);
     gst_element_set_state (input->parsebin, GST_STATE_NULL);
@@ -1505,6 +1508,7 @@ gst_decodebin3_handle_message (GstBin * bin, GstMessage * message)
         handle_stream_collection (dbin, collection,
             (GstElement *) GST_MESSAGE_SRC (message));
         posting_collection = TRUE;
+        gst_object_unref (collection);
         INPUT_UNLOCK (dbin);
       }