Fix error if onRemove in subsystem call :remove again

ecs/ecs.can

@@ -243,11 +243,14 @@ let system_mt = {
 	-- Returns all removed entities.
 	-- Complexity: O(1) per system.
 	remove = :(e, ...)
-		if e ~= nil and @_previous[e] then
+		if e ~= nil then
+			if @_previous[e] then
 				-- remove from subsystems
 				for _, s in ipairs(@systems) do
 					s:remove(e)
 				end
+			end
+			if @_previous[e] then -- recheck in case it was removed already from a subsystem onRemove callback
 				-- remove from linked list
 				let prev = @_previous[e]
 				if prev == true then
@@ -266,6 +269,7 @@ let system_mt = {
 				@entityCount -= 1
 				@onRemove(e[@name])
 			end
+		end
 		if ... then
 			return e, @remove(...)
 		else