Add missing flag initialization

The delay buffer `stopped` field was not initialized.

Since it practice the unique instance of sc_delay_buffer is initialized
in static memory, the flag was initialized to false as a side effect.

But with commit fd0f432e877153d83ed435474fb7b04e41de4269, in debug mode
only, the delay buffer was broken.

app/src/delay_buffer.c

@@ -132,6 +132,7 @@ sc_delay_buffer_frame_sink_open(struct sc_frame_sink *sink,
 
     sc_clock_init(&db->clock);
     sc_vecdeque_init(&db->queue);
+    db->stopped = false;
 
     if (!sc_frame_source_sinks_open(&db->frame_source, ctx)) {
         goto error_destroy_wait_cond;