s3:smb2_server: protect against integer wrap with "smb2 max credits = 65535"

s3:smb2_server: protect against integer wrap with "smb2 max credits = 65535"

Bug: https://bugzilla.samba.org/show_bug.cgi?id=9702

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Jan 29 14:58:40 CET 2015 on sn-devel-104

(similar to commit 8aed0fc38ae28cce7fd1a443844a865265fc719c)

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Thu Feb  5 23:44:30 CET 2015 on sn-devel-104