Fix crash when changing aux-panner state #10077

MixerStrip::show_send() may unset _panner if the send
has no panner. When then triggering an action that emits
`Pannable::automation_state_changed` (like un/link send panners)
of the main panner, Ardour crashed.

Note that PannerUI::set_panner already handed that case.

gtk2_ardour/panner_ui.cc

@@ -540,6 +540,9 @@ PannerUI::pan_automation_state_button_event (GdkEventButton *ev)
 void
 PannerUI::pan_automation_state_changed ()
 {
+	if (!_panner) {
+		return;
+	}
 	std::shared_ptr<Pannable> pannable (_panner->pannable());
 	pan_automation_state_button.set_label (GainMeterBase::short_astate_string(pannable->automation_state()));