fix bug for mis-converting uint64 value to an int64 one

[rosbit]

• If an uint64 value is greater than 0x7FFFFFFFFFFFFFFF(the max value of int64), the behavior is strange, sometimes a negative value will stored in db, or an error of "value out of range" occurs.
• In source files statement.go, session_convert.go, engine_cond.go, I found there're strange convertion like this:

  if !requiredField && fieldValue.Uint() == 0 {
    continue
  }
  t := int64(fieldValue.Uint())
  val = reflect.ValueOf(&t).Interface()
  

• I modify them, it works now.

  val = fieldValue.Interface()
  

[lunny]

Please send PR to https://gitea.com/xorm/xorm, We have moved there.