BUG/MEDIUM: listener/thread: bypass shards setting on failed thread resolution

BUG/MEDIUM: listener/thread: bypass shards setting on failed thread resolution

Aurélien reported that the BUG_ON(!new_ts.nbgrp) added in 2.8-dev3 by
commit 50440457e ("MEDIUM: config: restrict shards, not bind_conf to one
group each") can trigger on some invalid configs where the thread_set on
the "bind" line couldn't be resolved. The reason is that we still enter
the parsing loop (as it was done previously) and we possibly have no
group to work on (which was the purpose of this assertion). There we
need to bypass all this block on such a condition.

No backport is needed.