In ipcomp6_input(), check 'md' not 'm' after a call to m_pulldown(): 'm'

may be a stale pointer at this point, and we're interested in whether or
not m_pulldown() failed.

Noticed by:	Coverity Prevent analysis tool
MFC after:	3 days

sys/netinet6/ipcomp_input.c

@@ -258,7 +258,7 @@ ipcomp6_input(mp, offp, proto)
 	off = *offp;
 
 	md = m_pulldown(m, off, sizeof(*ipcomp), NULL);
-	if (!m) {
+	if (!md) {
 		m = NULL;	/* already freed */
 		ipseclog((LOG_DEBUG, "IPv6 IPComp input: assumption failed "
 		    "(pulldown failure)\n"));