tcp: Blackbox logging and tcp accounting together can cause a crash.

If you currently turn BB logging on and in combination have TCP Accounting on we can get a
crash where we have no NULL check and we run out of memory. Also lets make sure we
don't do a divide by 0 in calculating any BB ratios.

Reviewed by: tuexen
Sponsored by: Netflix Inc
Differential Revision:https://reviews.freebsd.org/D39622