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.
_waiter_create(uint32_t jobid)
{
	struct waiter *wp = xmalloc(sizeof(struct waiter));

	wp->jobid = jobid;
	wp->thd   = pthread_self();

	return wp;
}

_waiter_create(uint32_t jobid)
{
	struct waiter *wp = xmalloc(sizeof(struct waiter));

	wp->jobid = jobid;
	wp->thd   = pthread_self();

	return wp;
}
