Pull up following revision(s) (requested by maxv in ticket #667):

sys/netipsec/ipsec_input.c: revision 1.54

ipsec4_fixup_checksum calls m_pullup, so don't forget to do mtod() again,
to prevent use-after-free.

In fact, the m_pullup call is never reached: it is impossible for 'skip'
to be zero in this function, so add an XXX for now.

(martin)