unit_tactical_ai.lua: ops, fix crash

GetUnitIsDead() return NIL after unit long died and return FALSE if unit is alive @_@ (both is 'false')
Merge pull request #8 from xponen/master

unit_tactical_ai.lua: ops, fix crash
Merge pull request #9 from GoogleFrog/master

Improved Defense Ranges performance. It now uses Draw Lists.
Merge branch 'master' of https://github.com/ZeroK-RTS/Zero-K
Cleanup and performance improvement of Ghost Site by jK.