syscall: return error if GetQueuedCompletionStatus truncates key

This function has the wrong signature, so return an error when that
actually might lead to unexpected results. Users should switch to
x/sys/windows for the real version of this function.

Updates #44538.

Change-Id: I4d1f3d1e380815733ecfea683f939b1d25dcc32a
Reviewed-on: https://go-review.googlesource.com/c/go/+/296154


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>