s3: smbpasswd - fix crashes on invalid input.

get_pass can return NULL on error. Ensure that
this is always the case and fix all callers to cope
(some already did).

Reported by Joonas Kuorilehto <joneskoo@codenomicon.com>

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10320

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Mon Dec 16 15:17:58 CET 2013 on sn-devel-104

(cherry picked from commit ef5a3bedab74420baf0c653cf8e304fe6c2a13b4)

Autobuild-User(v4-1-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-1-test): Sat Jan 25 18:16:55 CET 2014 on sn-devel-104