fix: begin transaction fail, rollback panic

[zhouit]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

fix begin transaction fail, rollback panic

User Case Description

tx = db.Begin()  // when begin error

err := tx.Where(sql).Update(m).Err
if err = nil{
  tx.Rollback()   // will panic
  return err
}

tx.Commit()