proxysink: event_function needs to handle the event when it is disconnecetd from proxysrc

without this a disconneted proxysink fail when goes to play with error:

 Internal data stream error.
 streaming stopped, reason error (-5)

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1508>

gst/proxy/gstproxysink.c

@@ -226,8 +226,10 @@ gst_proxy_sink_sink_event (GstPad * pad, GstObject * parent, GstEvent * event)
       self->pending_sticky_events = TRUE;
       ret = TRUE;
     }
-  } else
+  } else {
     gst_event_unref (event);
+    ret = TRUE;
+  }
 
   return ret;
 }