rtsold: Fix a buffer leak if we fail to read the default router sysctl

MFC after:	1 week
Reported by:	Kevin Day <[email protected]>

Author: markjdb

usr.sbin/rtsold/cap_sendmsg.c

@@ -151,8 +151,10 @@ probe_defrouters(uint32_t ifindex, uint32_t linkid)
 	buf = malloc(len);
 	if (buf == NULL)
 		return (-1);
-	if (sysctl(mib, nitems(mib), buf, &len, NULL, 0) < 0)
+	if (sysctl(mib, nitems(mib), buf, &len, NULL, 0) < 0) {
+		free(buf);
 		return (-1);
+	}
 	ep = (struct in6_defrouter *)(void *)(buf + len);
 	for (p = (struct in6_defrouter *)(void *)buf; p < ep; p++) {
 		if (ifindex != p->if_index)