tcp: avoid useless calculations in tcp_account_for_send() when len is 0

[gallatin]

Summary

When the length is 0, the retransmit rate could not have changed, so avoid doing math to determine if we need to move a TLS connection from hardware to software.

This also fixes a potential bug when switching from an old or buggy TCP stack which does not call into tcp_account_for_send(). In this case, the first call into tcp_account_for_send() will find the t_sndbytes and t_snd_rxt_bytes as 0, and if that first call is for a rexmit, we'll hit a divide-by-zero bug unless we guard against a zero length.