Fix bug in the test of file descriptor of current WAL file in pg_receivexlog.

In pg_receivexlog, in order to check whether the current WAL file is
being opened or not, its file descriptor has to be checked against -1
as an invalid value. But, oops, 7900e94 added the incorrect test
checking the descriptor against 1. This commit fixes that bug.

Back-patch to 9.4 where the bug was added.

Spotted by Magnus Hagander

src/bin/pg_basebackup/receivelog.c

@@ -1021,7 +1021,7 @@ ProcessKeepaliveMsg(PGconn *conn, char *copybuf, int len,
 	if (replyRequested && still_sending)
 	{
 		if (reportFlushPosition && lastFlushPosition < blockpos &&
-			walfile != 1)
+			walfile != -1)
 		{
 			/*
 			 * If a valid flush location needs to be reported,