Fix thinko in logical decoding of commit-prepared records.

The decoding of prepared transaction commits accidentally used the XID of
the transaction performing the COMMIT PREPARED, not the XID of the prepared
transaction. Before bb38fb0d43 that lead to those transactions not being
decoded, afterwards to a assertion failure.

src/backend/replication/logical/decode.c

@@ -225,7 +225,7 @@ DecodeXactOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf)
 				subxacts = (TransactionId *) &(xlrec->xnodes[xlrec->nrels]);
 				invals = (SharedInvalidationMessage *) &(subxacts[xlrec->nsubxacts]);
 
-				DecodeCommit(ctx, buf, r->xl_xid, xlrec->dbId,
+				DecodeCommit(ctx, buf, prec->xid, xlrec->dbId,
 							 xlrec->xact_time,
 							 xlrec->nsubxacts, subxacts,
 							 xlrec->nmsgs, invals);