txnkv: fix a bug that commitTSUpperBoundCheck err not handled (#564)

Signed-off-by: tiancaiamao <tiancaiamao@gmail.com>

Co-authored-by: Yilin Chen <sticnarf@gmail.com>

txnkv/transaction/2pc.go

@@ -1547,6 +1547,7 @@ func (c *twoPhaseCommitter) execute(ctx context.Context) (err error) {
 		if !c.txn.commitTSUpperBoundCheck(commitTS) {
 			err = errors.Errorf("session %d check commit ts upper bound fail, txnStartTS: %d, comm: %d",
 				c.sessionID, c.startTS, c.commitTS)
+			return err
 		}
 	}