monitor.c: Fix infinite loop in monitor's auto-complete

The QEMU monitor enters an infinite loop when trying to auto-complete commands
that accept only optional parameters. The commands currently affected by this
issue are 'info registers' and 'info mtree'.

Reported-by: Dimitris Karagkasidis <t.pagef.lt@gmail.com>
Fixes: 48fe86f6
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20180213125143.23488-1-dgilbert@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>