tcp: Avoid division by zero when KERN_TLS is enabled in tcp_account_for_send().

If the "len" variable is non-zero, we can assume that the sum of
"tp->t_snd_rxt_bytes + tp->t_sndbytes" is also non-zero.

It is also assumed that the 64-bit byte counters will never wrap around.

Differential Revision: https://reviews.freebsd.org/D31959
Reviewed by: gallatin, rrs and tuexen
Found by: "I told you so", also called hselasky
MFC after: 1 week
Sponsored by: NVIDIA Networking