calendar-monitor: Fix critical in finalize

self->thread can be NULL if gcal_calendar_monitor_set_range() was never
called prior to finalize. This is fine, so just check for NULL here
using g_clear_pointer().

src/core/gcal-calendar-monitor.c

@@ -996,7 +996,7 @@ gcal_calendar_monitor_finalize (GObject *object)
 
   g_cancellable_cancel (self->cancellable);
   notify_view_thread (self, QUIT);
-  g_thread_join (self->thread);
+  g_clear_pointer (&self->thread, g_thread_join);
 
   remove_all_events (self);