fallbacksrc: Don't lock CustomSource state lock twice when exposing pads

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2474>

utils/fallbackswitch/src/fallbacksrc/custom_source/imp.rs

@@ -504,10 +504,13 @@ impl CustomSource {
                     .filter(|p| p.is_selected)
                     .map(|p| p.stream.stream_id().unwrap())
                     .collect::<Vec<_>>();
+                drop(state);
                 if let Some(message) = self.handle_stream_selection(selected_ids) {
                     let _ = self.obj().post_message(message);
                 }
             }
+        } else {
+            drop(state);
         }
 
         if is_exposed {