zetacore observer : add validation for negative ballot maturity blocks

[kingpinXD]

The validation for Ballot maturity blocks should include a check for negative values. We should not allow a negative value to be set for this

func validateBallotMaturityBlocks(i interface{}) error {
	v, ok := i.(int64)
	if !ok {
		return fmt.Errorf("invalid parameter type: %T", i)
	}
        if v < 0 {
               return an error
       }
	return nil
}

[skosito]

closed with #2100