mshtml: Only release the interface if it exists.

dlls/mshtml/persist.c

@@ -268,7 +268,7 @@ static void set_downloading_proc(task_t *_task)
         IDropTarget *drop_target = NULL;
 
         hres = IDocHostUIHandler_GetDropTarget(doc->hostui, NULL /* FIXME */, &drop_target);
-        if(SUCCEEDED(hres)) {
+        if(SUCCEEDED(hres) && drop_target) {
             FIXME("Use IDropTarget\n");
             IDropTarget_Release(drop_target);
         }