Fix integer overflow in debug message of walreceiver

The message tries to tell the replication apply delay which fails if
the first WAL record is not applied yet. Fix is, instead of telling
overflowed minus numeric, showing "N/A" which indicates that the delay
data is not yet available. Problem reported by me and patch by
Fabrízio de Royes Mello.

Back patched to 9.4, 9.3 and 9.2 stable branches (9.1 and 9.0 do not
have the debug message).

src/backend/replication/walreceiver.c

@@ -793,15 +793,26 @@ ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime)
 	{
 		char	   *sendtime;
 		char	   *receipttime;
+		int			applyDelay;
 
 		/* Copy because timestamptz_to_str returns a static buffer */
 		sendtime = pstrdup(timestamptz_to_str(sendTime));
 		receipttime = pstrdup(timestamptz_to_str(lastMsgReceiptTime));
-		elog(DEBUG2, "sendtime %s receipttime %s replication apply delay %d ms transfer latency %d ms",
-			 sendtime,
-			 receipttime,
-			 GetReplicationApplyDelay(),
-			 GetReplicationTransferLatency());
+		applyDelay = GetReplicationApplyDelay();
+
+		/* apply delay is not available */
+		if (applyDelay == -1)
+			elog(DEBUG2, "sendtime %s receipttime %s replication apply delay (N/A) transfer latency %d ms",
+				 sendtime,
+				 receipttime,
+				 GetReplicationTransferLatency());
+		else
+			elog(DEBUG2, "sendtime %s receipttime %s replication apply delay %d ms transfer latency %d ms",
+				 sendtime,
+				 receipttime,
+				 applyDelay,
+				 GetReplicationTransferLatency());
+
 		pfree(sendtime);
 		pfree(receipttime);
 	}

src/backend/replication/walreceiverfuncs.c

@@ -241,7 +241,8 @@ GetWalRcvWriteRecPtr(XLogRecPtr *latestChunkStart)
 }
 
 /*
- * Returns the replication apply delay in ms
+ * Returns the replication apply delay in ms or -1
+ * if the apply delay info is not available
  */
 int
 GetReplicationApplyDelay(void)
@@ -255,6 +256,8 @@ GetReplicationApplyDelay(void)
 	long		secs;
 	int			usecs;
 
+	TimestampTz	chunckReplayStartTime;
+
 	SpinLockAcquire(&walrcv->mutex);
 	receivePtr = walrcv->receivedUpto;
 	SpinLockRelease(&walrcv->mutex);
@@ -264,7 +267,12 @@ GetReplicationApplyDelay(void)
 	if (XLByteEQ(receivePtr, replayPtr))
 		return 0;
 
-	TimestampDifference(GetCurrentChunkReplayStartTime(),
+	chunckReplayStartTime = GetCurrentChunkReplayStartTime();
+
+	if (chunckReplayStartTime == 0)
+		return -1;
+
+	TimestampDifference(chunckReplayStartTime,
 						GetCurrentTimestamp(),
 						&secs, &usecs);