Avoid a buffer overwrite in rip6_output() when computing the checksum

as requested by the user via the IPPROTO_IPV6 level socket option
IPV6_CHECKSUM. The check if there are enough bytes in the packet to
store the checksum at the requested offset was wrong by 1.

Reviewed by:		bz@
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D19967

sys/netinet6/raw_ip6.c

@@ -495,7 +495,7 @@ rip6_output(struct mbuf *m, struct socket *so, ...)
 			off = offsetof(struct icmp6_hdr, icmp6_cksum);
 		else
 			off = in6p->in6p_cksum;
-		if (plen < off + 1) {
+		if (plen < off + 2) {
 			error = EINVAL;
 			goto bad;
 		}