pf: check anchor termination before stripping slashes

[ojanerif_amd.com]

Summary

pfr_fix_anchor() strips leading slashes before checking that the
anchor is NUL terminated. With an anchor made only of slashes the
scan runs past the end of the buffer and siz - off underflows in the
bcopy().

Check termination first. 1da3c0ca5b1 only did this for
DIOCRCLRTABLES. This covers the other callers. Add test cases for
DIOCRADDTABLES, DIOCRDELTABLES and DIOCRSETTFLAGS.

Sponsored by: Advanced Micro Devices, Inc.
Signed-off-by: Osvaldo Janeri Filho <[e-mail]>

Test Plan

kyua test in tests/sys/netpfil/pf (table, anchor, ioctl, names) passes
with the patch, including the new cases.

[kp]

Coincidentally I have a very similar patch I wrote yesterday.
I’ll figure out if I like yours or mine better when I’m back next week.