manager: Avoid reentrancy

While deleting the calendar, we may run into a situation
where the GcalCalendar instance is destroyed before the
GcalManager:calendar-removed signal is emitted, causing
us to emit the signal with a destroyed object.

Prevent that by increasing the ref count before emiting
the signal.

src/core/gcal-manager.c

@@ -153,7 +153,7 @@ static void
 remove_source (GcalManager  *self,
                ESource      *source)
 {
-  GcalCalendar *calendar;
+  g_autoptr (GcalCalendar) calendar = NULL;
 
   GCAL_ENTRY;
 
@@ -161,6 +161,7 @@ remove_source (GcalManager  *self,
   g_return_if_fail (E_IS_SOURCE (source));
 
   calendar = g_hash_table_lookup (self->clients, source);
+  g_object_ref (calendar);
 
   e_cal_data_model_remove_client (self->e_data_model,
                                   e_source_get_uid (source));