monitor: Plug memory leak on QMP error

Leak introduced in commit 8a4f501c..710aec91, v2.4.0.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1446117309-15322-1-git-send-email-armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>

monitor.c

@@ -3907,6 +3907,7 @@ static void handle_qmp_command(JSONMessageParser *parser, QList *tokens)
 err_out:
     monitor_protocol_emitter(mon, data, local_err);
     qobject_decref(data);
+    error_free(local_err);
     QDECREF(input);
     QDECREF(args);
 }