Bug#20615

  "plugin name lost when loading an already-loaded name"
  Do not delete existing plugin when loading a new plugin which has the
  same name.


sql/sql_plugin.cc:
  Bug#20615
    Don't delete the plugin if it wasn't added.

sql/sql_plugin.cc

@@ -864,8 +864,8 @@ my_bool mysql_install_plugin(THD *thd, const LEX_STRING *name, const LEX_STRING
   DBUG_RETURN(FALSE);
 deinit:
   plugin_deinitialize(tmp);
-err:
   plugin_del(tmp);
+err:
   rw_unlock(&THR_LOCK_plugin);
   DBUG_RETURN(TRUE);
 }