libalias: add another check to previous change

If UseLink() returns NULL, it is possible that Deletelink()
has already freed "grp", so check it out carefully.

PR:		269770
Reported by:	Peter Much
X-MFC-With:	8132e95909

sys/netinet/libalias/alias_db.c

@@ -875,6 +875,9 @@ _FindLinkIn(struct libalias *la, struct in_addr dst_addr,
 				if (found != NULL)
 					return (found);
 				/* link expired */
+				grp = StartPointIn(la, alias_addr, alias_port, link_type, 0);
+				if (grp == NULL)
+					return (NULL);
 				break;
 			}
 		}