Bug #57559: Crash in Client plugin api

The plugin code was releasing the plugin and 
only then was reporting an error referencing it.
Fixed by first reporting an error and then freeing up
the plugin.

sql-common/client_plugin.c

@@ -176,11 +176,11 @@ add_plugin(MYSQL *mysql, struct st_mysql_client_plugin *plugin, void *dlhandle,
   if (plugin->deinit)
     plugin->deinit();
 err1:
-  if (dlhandle)
-    dlclose(dlhandle);
   set_mysql_extended_error(mysql, CR_AUTH_PLUGIN_CANNOT_LOAD, unknown_sqlstate,
                            ER(CR_AUTH_PLUGIN_CANNOT_LOAD), plugin->name,
                            errmsg);
+  if (dlhandle)
+    dlclose(dlhandle);
   return NULL;
 }