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: NIdo Schimmel <idosch@idosch.org>
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>