hone: calling ipv6_skip_exthdr with NULL frag_offp hangs a system.

with icmpv6 destination unreachable messages, ipv6_skip_exthdr is called
with frag_offp set to NULL. Unfortunately, at least on 3.11, ipv6_skip_exthdr
can't handle a NULL argument and the system hangs.

You can test this by loading the packet and event modules (no need to open
/dev/hone or /dev/honet) and running (assuming nothing is listening on 9999/upd).

$ echo "test" | nc -6 -u ::1 9999