pg_rep_state: Take into account ready_to_display for WAL receiver

This flag will be set to true only once the data of the WAL receiver can
be displayed to the user, so return NULL if this is not the case yet in
the function showing the WAL receiver status. In Postgres 10, there is
already an in-core function doing this work, perhaps I should just plug in
this module into that, or simply remove it...

pg_rep_state/pg_rep_state.c

@@ -154,6 +154,7 @@ pg_wal_receiver_state(PG_FUNCTION_ARGS)
 	TimeLineID	receiveStartTLI, receivedTLI;
 	TimestampTz	lastMsgSendTime, lastMsgReceiptTime, latestWalEndTime;
 	char	   *slotname;
+	bool		ready_to_display;
 	WalRcvState	walRcvState;
 	if (!superuser())
@@ -175,10 +176,11 @@ pg_wal_receiver_state(PG_FUNCTION_ARGS)
 	latestWalEndTime = walrcv->latestWalEndTime;
 	walRcvState = walrcv->walRcvState;
 	slotname = pstrdup(walrcv->slotname);
+	ready_to_display = walrcv->ready_to_display;
 	SpinLockRelease(&walrcv->mutex);
 	/* Leave if no WAL receiver */
-	if (pid == 0)
+	if (pid == 0 || !ready_to_display)
 		PG_RETURN_NULL();
 	/* Initialise values and NULL flags arrays */