tcp: prevent spurious empty segments and fix uncommon panic

[rscheff]

Summary

When receiving pure ACKs for previously sent data,
but no more data is available in the send buffer,
an spurious empty ACK would frequently be sent instead.

In the case of a retransmitted SYN not being sent due to
an internal error, the snd_una/snd_nxt accounting could
be off, leading to a panic. Pulling snd_nxt up to snd_una
prevents this from happening.

Reported by: [e-mail]

[cc]

Good catch!