ipfw: Check for errors from sooptcopyin() and sooptcopyout()

Note, it looks like this code may be unused since commit 4a77657cbc01
("ipfw: migrate ipfw to 32-bit size rule numbers"). In particular, it
looks like the ipfw_nat_*_ptr pointers are unused now.

Reviewed by: ae
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53068