[FIX] calendar: do not write end time before start time of an event

The end time of a calendar event is constrained to never be before the start
of te event.
However when updating events, one path would first update the start of the event
before updating the end of the event.
So in case where (s_1, e_1) is updated by (s_2, e_2),
with respectively s_i < e_i,
if s2 > e1 then the update would violate the constraint and thus crash
the calendar update.

opw 1854394

addons/calendar/models/calendar.py

@@ -816,8 +816,8 @@ class Meeting(models.Model):
                 startdate = startdate.astimezone(pytz.utc)  # Convert to UTC
                 meeting.start = fields.Datetime.to_string(startdate)
             else:
-                meeting.start = meeting.start_datetime
-                meeting.stop = meeting.stop_datetime
+                meeting.write({'start': meeting.start_datetime,
+                               'stop': meeting.stop_datetime})
 
     @api.depends('byday', 'recurrency', 'final_date', 'rrule_type', 'month_by', 'interval', 'count', 'end_type', 'mo', 'tu', 'we', 'th', 'fr', 'sa', 'su', 'day', 'week_list')
     def _compute_rrule(self):