Fix crash when closing session with VCA slaved solo

Session::destroy() calls drop_references(),
which leads to  InternalSend::send_from_going_away()
calling InternalSend::propagate_solo().

This looks up the SoloControl to test soloed_by_others(), incl.
and VCA maters. Those VCAs however may already have been destroyed,
and (weak pointer) _master.lock() fails.

libs/ardour/internal_send.cc

@@ -83,6 +83,9 @@ InternalSend::~InternalSend ()
 void
 InternalSend::propagate_solo ()
 {
+	if (_session.inital_connect_or_deletion_in_progress ()) {
+		return;
+	}
 	if (!_send_to || !_send_from) {
 		return;
 	}