Fix valgrind warning from deinitializing ha_ndbcluster, referencing

another handler from 'plugin_foreach'


sql/sql_plugin.cc:
  Remove plugin from plugin_array after it has been initialized to 
  avoid it being referenced. For example if a plugin deletes a THD object
  that will call 'ha_close_connection' wich will iterate over all storage
  engine plugins

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);
 
   }