fix an assert failure due to incorrect process locking when adding a send

git-svn-id: svn://localhost/ardour2/branches/3.0@11275 d708f5d6-7413-0410-9779-e7cbd77b26cf

libs/ardour/route.cc

@@ -2551,6 +2551,7 @@ Route::add_internal_return ()
 void
 Route::add_send_to_internal_return (InternalSend* send)
 {
+	Glib::Mutex::Lock lm (AudioEngine::instance()->process_lock ());
 	Glib::RWLock::ReaderLock rm (_processor_lock);
 
 	for (ProcessorList::const_iterator x = _processors.begin(); x != _processors.end(); ++x) {