Fix an oversight in ReorderBufferFinishPrepared.

We don't have anything to decode in a transaction if ReorderBufferTXN
doesn't exist by the time we decode the commit prepared. So don't create a
new ReorderBufferTXN here. This is an oversight in commit a271a1b5.

Reported-by: Markus Wanner
Discussion: https://postgr.es/m/dbec82e2-dbd7-95a2-c6b6-e488cbbdf853@bluegap.ch

src/backend/replication/logical/reorderbuffer.c

@@ -2679,7 +2679,7 @@ ReorderBufferFinishPrepared(ReorderBuffer *rb, TransactionId xid,
 	XLogRecPtr	prepare_end_lsn;
 	TimestampTz prepare_time;
 
-	txn = ReorderBufferTXNByXid(rb, xid, true, NULL, commit_lsn, false);
+	txn = ReorderBufferTXNByXid(rb, xid, false, NULL, commit_lsn, false);
 
 	/* unknown transaction, nothing to do */
 	if (txn == NULL)