Ensure cwnd doesn't shrink to zero with PRR

Under some circumstances, PRR may end up with a fully
collapsed cwnd when finalizing the loss recovery.

Reviewed By: transport, kbowling
Reported by: Liang Tian
MFC after: 5 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28780

(cherry picked from commit 853fd7a2e39802e46bd3d6476529796ac22412d9)