packet: validate address length if non-zero

Validate packet socket address length if a length is given. Zero
length is equivalent to not setting an address.

Fixes: 99137b78

 ("packet: validate address length")
Reported-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>