Fast MAI: bug fix for error when attacker runs into ambush

If the move of the attacker toward the target is interrupted by an
ambush, the attack action previously caused an error, because the
attacker did  not get to its target.

data/ai/micro_ais/cas/ca_fast_combat.lua

@@ -63,9 +63,11 @@ function ca_fast_combat:execution(ai, cfg, self)
     local unit = self.data.fast_combat_units[self.data.fast_combat_unit_i]
     AH.movefull_outofway_stopunit(ai, unit, self.data.fast_dst.x, self.data.fast_dst.y)
 
-    if unit and unit.valid and self.data.fast_target and self.data.fast_target.valid then
-        AH.checked_attack(ai, unit, self.data.fast_target)
-    end
+    if (not unit) or (not unit.valid) then return end
+    if (not self.data.fast_target) or (not self.data.fast_target.valid) then return end
+    if (H.distance_between(unit.x, unit.y, self.data.fast_target.x, self.data.fast_target.y) ~= 1) then return end
+
+    AH.checked_attack(ai, unit, self.data.fast_target)
 
     self.data.fast_combat_units[self.data.fast_combat_unit_i] = nil
 end