ipfw: guard against NULL deref with clat/plat prefixes without length

Found with:	Claude Code Sonnet 5
MFC after:	2 weeks

sbin/ipfw/nat64clat.c

@@ -208,6 +208,9 @@ nat64clat_create(const char *name, uint8_t set, int ac, char *av[])
 
 			if ((p = strchr(*av, '/')) != NULL)
 				*p++ = '\0';
+			else
+				errx(EX_USAGE,
+				    "Prefix length required: %s", *av);
 			if (inet_pton(AF_INET6, *av, &prefix) != 1)
 				errx(EX_USAGE,
 				    "Bad prefix: %s", *av);