TCP PRR: Patch div/0 in tcp_prr_partialack

Adding a safety net prior to the division in tcp_prr_partialack
function, which was missed in D28114.

Reviewed-by: tuexen, mm, @transport

MFC: 3 days

Differential Revision: https://reviews.freebsd.org/D28326

(cherry picked from commit 6a376af0cd212be4e16d013d35a0e2eec1dbb8ae)