Fix: memory leak in clip_buttons.cc

uiTemplateMarker could return early with a MarkerUpdateCb not freed,
now fixed.

Part of #120767

Pull Request: https://projects.blender.org/blender/blender/pulls/121101

source/blender/editors/space_clip/clip_buttons.cc

@@ -461,7 +461,7 @@ void uiTemplateMarker(uiLayout *layout,
                0,
                0,
                "");
-
+      MEM_freeN(cb);
       return;
     }