Adjust input value to WaitEventSetWait() in ExecAppendAsyncEventWait().

Adjust the number of events given to WaitEventSetWait() so that it
doesn't exceed the maximum number of events in the WaitEventSet given
to that function (set->nevents_space) in hopes of making the buildfarm
green.

Per valgrind failure report from Tom Lane and the buildfarm.

Author: Etsuro Fujita
Discussion: https://postgr.es/m/3411577.1617289776%40sss.pgh.pa.us

src/backend/executor/nodeAppend.c

@@ -1001,6 +1001,7 @@ ExecAppendAsyncEventWait(AppendState *node)
 	long		timeout = node->as_syncdone ? -1 : 0;
 	WaitEvent   occurred_event[EVENT_BUFFER_SIZE];
 	int			noccurred;
+	int			nevents;
 	int			i;
 
 	/* We should never be called when there are no valid async subplans. */
@@ -1022,8 +1023,9 @@ ExecAppendAsyncEventWait(AppendState *node)
 	}
 
 	/* Wait for at least one event to occur. */
+	nevents = Min(node->as_nasyncplans + 1, EVENT_BUFFER_SIZE);
 	noccurred = WaitEventSetWait(node->as_eventset, timeout, occurred_event,
-								 EVENT_BUFFER_SIZE, WAIT_EVENT_APPEND_READY);
+								 nevents, WAIT_EVENT_APPEND_READY);
 	FreeWaitEventSet(node->as_eventset);
 	node->as_eventset = NULL;
 	if (noccurred == 0)