zombie.lua: Fixed script failure due to uninitialized array index/indices.
The 'table filtering routine' stored results in the wrong array index/indices
if a previous result was filtered out (not stored).
Found and verfied with the 'tools/multipound/pound.sh' test.

human.lua: Removed the unused and erroneous 'table filtering routine'.

GameState::findVisibleUnits(): Minor code style changes to make it easier to read the code.

execScript() x 2: Return false if script failed (return value not used today).