How can I stop the negative number at project.task?

I want to stop that the field "remaining_hours" can go under 0...

I try to build a module for that, but how can I overwrite a function from the odoo basic module "project"?

example: 

    def _hours_get(self, cr, uid, ids, field_names, args, context=None):

        res = {}

        cr.execute("SELECT task_id, COALESCE(SUM(hours),0) FROM project_task_work WHERE task_id IN %s GROUP BY task_id",(tuple(ids),))

        hours = dict(cr.fetchall())

        for task in self.browse(cr, uid, ids, context=context):

            res[task.id] = {'effective_hours': hours.get(task.id, 0.0), 'total_hours': (task.remaining_hours or 0.0) + hours.get(task.id, 0.0)}

            res[task.id]['delay_hours'] = res[task.id]['total_hours'] - task.planned_hours

            res[task.id]['progress'] = 0.0

            if (task.remaining_hours < 0 ):

                task.planned_hours = 100.0

            if (task.remaining_hours + hours.get(task.id, 0.0)):

                res[task.id]['progress'] = round(min(100.0 * hours.get(task.id, 0.0) / res[task.id]['total_hours'], 99.99),2)

            # TDE CHECK: if task.state in ('done','cancelled'):

            if task.stage_id and task.stage_id.fold:

                res[task.id]['progress'] = 100.0

        return res

I think you can do this restriction by coding, i.e. in the write method of the task screen add a check that the final remaining hours must be greater than zero