mld6: initialize oifp to avoid bogus results/panics in edge cases

In certain cases (probably not during normal operation but observed in
the lab during development) ip6_ouput() could return without error
and ifpp (&oifp) not updated.
Given oifp was never initialized we would take the later branch
as oifp was not NULL, and when calling icmp6_ifstat_inc() we would
panic dereferencing a garbage pointer.
For code stability initialize oifp to NULL before first use to always
have a deterministic value and not rely on a called function to behave
and always and for ever do the work for us as we hope for.

MFC after:	3 days
Sponsored by:	Netflix

sys/netinet6/mld6.c

@@ -3159,6 +3159,7 @@ mld_dispatch_packet(struct mbuf *m)
 	mld = (struct mld_hdr *)(mtod(md, uint8_t *) + off);
 	type = mld->mld_type;
 
+	oifp = NULL;
 	error = ip6_output(m0, &mld_po, NULL, IPV6_UNSPECSRC, &im6o,
 	    &oifp, NULL);
 	if (error) {