core/calendar-monitor: Keep event alive while removing it

This might be the last reference to it, so keep it alive while
the signal is being emitted.

src/core/gcal-calendar-monitor.c

@@ -1022,8 +1022,12 @@ remove_all_events (GcalCalendarMonitor *self)
   g_hash_table_iter_init (&iter, self->shared.events);
   while (g_hash_table_iter_next (&iter, NULL, (gpointer*) &event))
     {
+      g_object_ref (event);
+
       g_hash_table_iter_remove (&iter);
       g_signal_emit (self, signals[EVENT_REMOVED], 0, event);
+
+      g_object_unref (event);
     }
 }