Fix v4l2 data race

The v4l2_sink implementation directly read the internal video_buffer
field "pending_frame_consumed", which is protected by the internal
video_buffer mutex. But this mutex was not locked, so reads were racy.

Lock using the v4l2_sink mutex in addition, and use a separate field to
avoid depending on the video_buffer internal data.

app/src/v4l2_sink.h

@@ -22,6 +22,7 @@ struct sc_v4l2_sink {
     sc_thread thread;
     sc_mutex mutex;
     sc_cond cond;
+    bool has_frame;
     bool stopped;
     bool header_written;