parsebin: Fix a potential stream collection leak

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5607>

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

@@ -3637,6 +3637,8 @@ retry:
     GST_WARNING_OBJECT (parsebin,
         "Currently, shutting down, aborting exposing");
     DYN_UNLOCK (parsebin);
+    if (fallback_collection)
+      gst_object_unref (fallback_collection);
     return FALSE;
   }