Add type check to nativefs.getDirectoryItems(dir)

Most incorrect types for 'dir' are caught deeper into the call stack, but passing nil causes a coredump. This type-check mirrors the behavior of love.filesystem.getDirectoryItems().

nativefs.lua

@@ -351,6 +351,9 @@ local function withTempMount(dir, fn, ...)
 end
 
 function nativefs.getDirectoryItems(dir)
+	if type(dir) ~= "string" then
+		error("bad argument #1 to 'getDirectoryItems' (string expected, got " .. type(dir) .. ")")
+	end
 	local result, err = withTempMount(dir, love.filesystem.getDirectoryItems)
 	return result or {}
 end