Fix building with WAL_DEBUG.

Now that the backup blocks are appended to the WAL record in xloginsert.c,
XLogInsert doesn't see them anymore and cannot remove them from the version
reconstructed for xlog_outdesc. This makes running with wal_debug=on more
expensive, as we now make (unnecessary) temporary copies of the backup
blocks, but it doesn't seem worth convoluting the code to keep that
optimization.

Reported by Alvaro Herrera.

src/backend/access/transam/xlog.c

@@ -1067,11 +1067,8 @@ XLogInsertRecord(XLogRecData *rdata, XLogRecPtr fpw_lsn)
 			/*
 			 * We have to piece together the WAL record data from the
 			 * XLogRecData entries, so that we can pass it to the rm_desc
-			 * function as one contiguous chunk. (but we can leave out any
-			 * extra entries we created for backup blocks)
+			 * function as one contiguous chunk.
 			 */
-			rdt_lastnormal->next = NULL;
-
 			initStringInfo(&recordbuf);
 			appendBinaryStringInfo(&recordbuf, (char *) rechdr, sizeof(XLogRecord));
 			for (; rdata != NULL; rdata = rdata->next)