fixed a kernel crash due to an improper removal of callout-timer

(ToDo: similar fix is necessary for other NDP-related callout-timers
 in netinet6/nd6*.c)

PR: kern/88725
MFC after: 1 month

sys/netinet6/mld6.c

@@ -637,6 +637,7 @@ in6_delmulti(in6m)
 		 * No remaining claims to this record; let MLD6 know
 		 * that we are leaving the multicast group.
 		 */
+		mld_stoptimer(in6m);
 		mld6_stop_listening(in6m);
 		ifma->ifma_protospec = NULL;
 		LIST_REMOVE(in6m, in6m_entry);