core/calendar-monitor: Clear shared rw-lock on finalize

In gcal_calendar_monitor_init(), self->shared.lock is initialized with
g_rw_lock_init(). On Linux/glibc, GRWLock allocates memory for the
underlying pthread_rwlock_t (56 bytes).

Because g_rw_lock_clear() was never called in gcal_calendar_monitor_finalize(),
every finalized GcalCalendarMonitor leaked its lock resources, failing the
LeakSanitizer CI build when test_calendar_monitor_expand_recurrences was added.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/808>

src/core/gcal-calendar-monitor.c

@@ -1327,6 +1327,8 @@ gcal_calendar_monitor_finalize (GObject *object)
   g_clear_pointer (&self->shared.filter, g_free);
   g_clear_pointer (&self->shared.range, gcal_range_unref);
 
+  g_rw_lock_clear (&self->shared.lock);
+
   G_OBJECT_CLASS (gcal_calendar_monitor_parent_class)->finalize (object);
 }