sched: Fix integer overflow for KIST

Ticket #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.