Fixed rare errors.

Consider possibility of nil or NULL being a value in the tables returned
from util.FindInSphere, util.FindInBox, and ents.GetAll.
Disable message beacon about force fields.

They aren't available so no point to have this.
Fix zs_noredeem convar not working.
Fix typo in GetNailOwner
Fix medical aura persisting when it shouldn't.
Merge branch 'master' of https://github.com/JetBoom/zombiesurvival