udp: fix udp checksum

we did not apply the special case to store 0xFFFF (-0)
in the checksum field when the checksum calculation
returned zero. we survived this for v4 as RFC768 states:

> If the computed checksum is zero, it is transmitted as
> all ones (the equivalent in one's complement arithmetic).
>
> An all zero transmitted checksum value means that the
> transmitter generated no checksum (for debuging or for
> higher level protocols that don't care).

for ipv6 however, the checksum is not optional and receivers
would drop packets with a zero checksum.

0 files changed, 0 insertions, 0 deletions