smb: client: allow parsing zero-length AV pairs

Zero-length AV pairs should be considered as valid target infos.
Don't skip the next AV pairs that follow them.

Cc: linux-cifs@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Fixes: 0e8ae9b9 ("smb: client: parse av pair type 4 in CHALLENGE_MESSAGE")
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Signed-off-by: Steve French <stfrench@microsoft.com>