Don't skip jobs in scontrol hold

Fix regression caused by commit bb8b4214.

Commit bb8b4214 introduced this in scontrol_hold():

-		job_msg.job_id_str = _next_job_id();
-		while (job_msg.job_id_str) {
+		while ((job_msg.job_id_str = _next_job_id())) {

But didn't take out _next_job_id() at the bottom of the while loop -
so _next_job_id() is called twice in a row, skipping every other job
in the list.

Bug 5902