Fix security issue in _prolog_error().

Fix security issue caused by insecure file path handling triggered by
the failure of a Prolog script. To exploit this a user needs to
anticipate or cause the Prolog to fail for their job.

(This commit is slightly different from the fix to the 15.08 branch.)

CVE-2016-10030.
static void _waiter_destroy(struct waiter *wp)
{
	xfree(wp);
}

static void _waiter_destroy(struct waiter *wp)
{
	xfree(wp);
}
