Fix WaitLatch(NULL) on Windows.

Further to commit 733fa9aa, on Windows when a latch is triggered but we
aren't currently waiting for it, we need to locate the latch's HANDLE
rather than calling ResetEvent(NULL).

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reported-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://postgr.es/m/CAEudQArTPi1YBc%2Bn1fo0Asy3QBFhVjp_QgyKG-8yksVn%2ByRTiw%40mail.gmail.com

src/backend/storage/ipc/latch.c

@@ -1835,7 +1835,11 @@ WaitEventSetWaitBlock(WaitEventSet *set, int cur_timeout,
 
 	if (cur_event->events == WL_LATCH_SET)
 	{
-		if (!ResetEvent(set->latch->event))
+		/*
+		 * We cannot use set->latch->event to reset the fired event if we
+		 * aren't waiting on this latch now.
+		 */
+		if (!ResetEvent(set->handles[cur_event->pos + 1]))
 			elog(ERROR, "ResetEvent failed: error code %lu", GetLastError());
 
 		if (set->latch && set->latch->is_set)