[svn] Frenzy and effStalker bug

[Hirumakai]

I ran into a unique snow giant with Frenzy, but no stalker talent. When it tried to frenzy it produced a Lua error.

Basically it couldn't index a local variable effStalker, as it was a nil.

I've attached a patch file, which basically changes line 109 in slaughter.lua to have a if effStalker clause before going into the core.fov.distance function.