fix bug in feeding code

wesnoth.current.event_context might not contain all information if the die event was triggered by wml ([fire_event]), this made wesnoth.get_unit error (invalid argument)

data/lua/feeding.lua

@@ -7,6 +7,11 @@ local on_event = wesnoth.require("on_event")
 -- The feeding event code
 on_event("die", function()
 	local ec = wesnoth.current.event_context
+
+	if not ec.x1 or not ec.y1 or not ec.x2 or not ec.y2 then
+		return
+	end
+
 	local u_killer = wesnoth.get_unit(ec.x2, ec.y2)
 	local u_victim = wesnoth.get_unit(ec.x1, ec.y1)