Pass more than the first XLogRecData entry to rm_desc, with WAL_DEBUG.

If you compile with WAL_DEBUG and enable it with wal_debug=on, we used to
only pass the first XLogRecData entry to the rm_desc routine. I think the
original assumprion was that the first XLogRecData entry contains all the
necessary information for the rm_desc routine, but that's a pretty shaky
assumption. At least standby_redo didn't get the memo.

To fix, piece together all the data in a temporary buffer, and pass that to
the rm_desc routine.

It's been like this forever, but the patch didn't apply cleanly to
back-branches. Probably wouldn't be hard to fix the conflicts, but it's
not worth the trouble.

src/backend/access/transam/xlog.c

@@ -1262,8 +1262,23 @@ begin:;
 		xlog_outrec(&buf, rechdr);
 		if (rdata->data != NULL)
 		{
+			StringInfoData recordbuf;
+
+			/*
+			 * 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)
+			 */
+			rdt_lastnormal->next = NULL;
+
+			initStringInfo(&recordbuf);
+			for (;rdata != NULL; rdata = rdata->next)
+				appendBinaryStringInfo(&recordbuf, rdata->data, rdata->len);
+
 			appendStringInfoString(&buf, " - ");
-			RmgrTable[rechdr->xl_rmid].rm_desc(&buf, rechdr->xl_info, rdata->data);
+			RmgrTable[rechdr->xl_rmid].rm_desc(&buf, rechdr->xl_info, recordbuf.data);
+			pfree(recordbuf.data);
 		}
 		elog(LOG, "%s", buf.data);
 		pfree(buf.data);