Prevent underflows in tp->snd_wnd if the remote side ACKs more than
tp->snd_wnd. This can happen, for example, when the remote side responds to
a window probe by ACKing the one byte it contains.

Differential Revision: https://reviews.freebsd.org/D5625
Reviewed by: hiren
Obtained from: Juniper Networks (earlier version)
MFC after: 2 weeks
Sponsored by: Juniper Networks