sched: Fix integer overflow for KIST
Ticket legacy/trac#24423 (moved) found a possible integer overflow with:
tcp_space = (ent->cwnd - ent->unacked) * (int64_t)ent->mss;
... if we ever end up with
cwnd being smaller than
unacked. I've observed this on a relay leading to huge values for
tcp_space which leads to the wrong TCP limit for the channel.
Any overflow should result in
tcp_space = 0 in practice.