Fix the same-rel optimization when creating WAL records.

prev_regbuf was never set, and therefore the same-rel flag was never set on
WAL records.

Report and fix by Zhanq Zq

src/backend/access/transam/xloginsert.c

@@ -667,10 +667,10 @@ XLogRecordAssemble(RmgrId rmid, uint8 info,
 		{
 			samerel = true;
 			bkpb.fork_flags |= BKPBLOCK_SAME_REL;
-			prev_regbuf = regbuf;
 		}
 		else
 			samerel = false;
+		prev_regbuf = regbuf;
 
 		/* Ok, copy the header to the scratch buffer */
 		memcpy(scratch, &bkpb, SizeOfXLogRecordBlockHeader);