[8.0 calendar] Fix a bug on my db

Hi,

This modification correct that bug :


  File "/home/odoo/openerp-8.0-1218/openerp/api.py", line 235, in wrapper
    return new_api(self, *args, **kwargs)
  File "/home/odoo/openerp-8.0-1218/openerp/api.py", line 552, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **kwargs)
  File "/home/odoo/openerp-8.0/openerp/addons/calendar/calendar.py", line 1555, in write
    data = self.read(cr, uid, event_id, ['start', 'stop', 'rrule', 'duration'])
  File "/home/odoo/openerp-8.0-1218/openerp/api.py", line 237, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/odoo/openerp-8.0/openerp/addons/calendar/calendar.py", line 1631, in read
    select = map(lambda x: (x, calendar_id2real_id(x)), select)
  File "/home/odoo/openerp-8.0/openerp/addons/calendar/calendar.py", line 1631, in <lambda>
    select = map(lambda x: (x, calendar_id2real_id(x)), select)
  File "/home/odoo/openerp-8.0/openerp/addons/calendar/calendar.py", line 44, in calendar_id2real_id
    return int(real_id)
ValueError: invalid literal for int() with base 10: ''