HEIMDAL:kdc: fix dh->q allocation check in get_dh_param()

HEIMDAL:kdc: fix dh->q allocation check in get_dh_param()

Thanks to Doug Nazar <nazard@nazar.ca> for spotting this!

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12986

Signed-off-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from heimdal commit a79b59ba27070a015479e8d981b7e685dbe34310)
Reviewed-by: Andreas Schneider <asn@samba.org>
(cherry picked from commit 183e5d1e3dc306491c06f94c8c98e4882c64bc27)

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Sun Dec 24 01:15:07 CET 2017 on sn-devel-144