Put back important code that went missing in the 8.3 merge.

Without this, a standby server gets stuck in an infinite loop, trying to
recover from a file in pg_xlog. It will also quickly fill all available disk
space by printing "record with zero len" messages in the log.

Reported off-list by Michael Kent, analysis by Amil Khanzada, Jimmy Yih, and
others.

src/backend/access/transam/xlog.c

@@ -4422,6 +4422,12 @@ next_record_is_invalid:
 
 	failedSources |= readSource;
 
+	if (readFile >= 0)
+	{
+		close(readFile);
+		readFile = -1;
+	}
+
 	nextRecord = NULL;
 
 	/* In standby-mode, keep trying */