Fix valgrind warning from deinitializing ha_ndbcluster, referencing

another handler from 'plugin_foreach'

sql/sql_plugin.cc

@@ -804,6 +804,7 @@ void plugin_shutdown(void)
     struct st_plugin_int *tmp= dynamic_element(&plugin_array, i,
                                                struct st_plugin_int *);
     plugin_deinitialize(tmp);
+    plugin_del(tmp);
 
   }