Fix type for write(2) return variable.

A little fix in xwrite().

>From 3727d2e3344b57128ab51c7839795204f1f839ff Mon Sep 17 00:00:00 2001
From: Quentin Rameau <[email protected]>
Date: Fri, 24 Jul 2015 11:40:46 +0200
Subject: [PATCH] Fix type for write(2) return variable.

The allocated lengh of s fits into an integer so we can safely use
ssize_t here.