s3/libsmb: Avoid potential smbpanic calling parse_user_quota_list.

s3/libsmb: Avoid potential smbpanic calling parse_user_quota_list.

Calling parse_user_quota_list with a NULL buffer can cause a panic, while
this shouldn't happen, I managed to trigger this with an early implementation
of SMB2 quota support in smbd which didn't pass back NT_STATUS_NO_MORE_ENTRIES
when handling a SMB2_0_INFO_QUOTA GETINFO message.
OTHOH the Windows client handled the same situation gracefully.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13553

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>