[libc] Slight optimization for small send_n / recv_n buffers (#218551)

Summary:
Adds a convergent short-circuit condition that lets the compiler
optimize out this loop better in cases where the size is known. Without
this sometimes the compiler could not figure out that a constant size
did not need the extra loop and the backedges would remain.
GitOrigin-RevId: fa1d193efd36888ea87233540e677e51f43eb279