Closes #24596.

"ours" merge to avoid taking the 030 version of 23862.

(I do not know why this one is happening)

(It can't fail because the tor_malloc*() family of functions can
never return NULL)

Found with STACK

(It can't fail because the tor_malloc*() family of functions can
never return NULL)

Found with STACK.

In KIST, we could have a small congestion window value than the unacked
packets leading to a integer overflow which leaves the tcp_space value to be
humongous.

This has no security implications but it results in KIST scheduler allowing to
send cells on a potentially saturated connection.

Found by #24423. Fixes #24590.

Signed-off-by: David Goulet <dgoulet@torproject.org>