Fast Micro AI: make code robust against events removing units

The previous version caused errors under some circumstances when an
event removed units other than the attacker during or immediately after
an AI attack.

data/ai/micro_ais/cas/ca_fast_combat.lua

@@ -79,10 +79,10 @@ function ca_fast_combat:evaluation(cfg, data)
 
     for i = #data.fast_combat_units,1,-1 do
         local unit = data.fast_combat_units[i]
-        local unit_info = FAU.get_unit_info(unit, data.gamedata)
-        local unit_copy = FAU.get_unit_copy(unit.id, data.gamedata)
 
-        if (unit.attacks_left > 0) and (#unit.attacks > 0) then
+        if unit and unit.valid and (unit.attacks_left > 0) and (#unit.attacks > 0) then
+            local unit_info = FAU.get_unit_info(unit, data.gamedata)
+            local unit_copy = FAU.get_unit_copy(unit.id, data.gamedata)
             local attacks = AH.get_attacks({ unit }, { include_occupied = cfg.include_occupied_attack_hexes, viewing_side = viewing_side })
 
             if (#attacks > 0) then