Fix possible race condition on video_buffer end

The video_buffer thread clears the queue once it is stopped, but new
frames might still be pushed asynchronously.

To avoid the problem, do not push any frame once the video_buffer is
stopped.

app/src/video_buffer.c

@@ -99,6 +99,8 @@ run_buffering(void *data) {
     }
 
 stopped:
+    assert(vb->b.stopped);
+
     // Flush queue
     while (!sc_vecdeque_is_empty(&vb->b.queue)) {
         struct sc_video_buffer_frame *p = sc_vecdeque_popref(&vb->b.queue);
@@ -206,6 +208,11 @@ sc_video_buffer_push(struct sc_video_buffer *vb, const AVFrame *frame) {
 
     sc_mutex_lock(&vb->b.mutex);
 
+    if (vb->b.stopped) {
+        sc_mutex_unlock(&vb->b.mutex);
+        return false;
+    }
+
     sc_tick pts = SC_TICK_FROM_US(frame->pts);
     sc_clock_update(&vb->b.clock, sc_tick_now(), pts);
     sc_cond_signal(&vb->b.wait_cond);