Wall spell fixes

This adds a few protections to fix abuses.

- If a wall spell's caster is hiding in a safe zone, the wall does
nothing
- If a wall spell's caster is logged off, the wall does nothing
- If the victim is an innocent with less than 60 base max health, the
wall does nothing

This should help mitigate the practice of slaughtering newbies without
risk, using firewalls that kill low HP characters instantly while the PK
is sitting somewhere safe.