Pull up following revision(s) (requested by maxv in ticket #799):

sys/netipsec/ipsec_output.c: revision 1.75
sys/netipsec/ipsec_output.c: revision 1.67

Strengthen this check, to make sure there is room for an ip6_ext structure.
Seems possible to crash m_copydata here (but I didn't test more than that).

Fix the checks in compute_ipsec_pos, otherwise m_copydata could crash. I
already fixed half of the problem two months ago in rev1.67, back then I
thought it was not triggerable because each packet we emit is guaranteed
to have correctly formed IPv6 options; but it is actually triggerable via
IPv6 forwarding, we emit a packet we just received, and we don't sanitize
its options before invoking IPsec.

Since it would be wrong to just stop the iteration and continue the IPsec
processing, allow compute_ipsec_pos to fail, and when it does, drop the
packet entirely.

(martin)