We needn't check "m" for NULL here because "off" should be within

the mbuf chain.  If we ever get a buggy caller, a bogus "off" should
be caught by the sanity check at the function entry.  Null "m" here
means a very unusual condition of a totally broken mbuf chain (wrong
m_pkthdr.len or whatever), so we can just page fault later.

Found by:	Coverity Prevent(tm)
CID:		825

sys/netinet6/in6_cksum.c

@@ -153,7 +153,7 @@ in6_cksum(struct mbuf *m, u_int8_t nxt, u_int32_t off, u_int32_t len)
 	/*
 	 * Secondly calculate a summary of the first mbuf excluding offset.
 	 */
-	while (m != NULL && off > 0) {
+	while (off > 0) {
 		if (m->m_len <= off)
 			off -= m->m_len;
 		else