v4l2videodec: Check stop in flush() to avoid race condition.

Backward playback will drain and flush every frame. Stop playback
when backward playback have race condition between exit thread and
streaming thread flush. Add one check to avoid it.

Fixes #639

sys/v4l2/gstv4l2videodec.c

@@ -310,6 +310,9 @@ gst_v4l2_video_dec_flush (GstVideoDecoder * decoder)
     GST_VIDEO_DECODER_STREAM_LOCK (decoder);
   }
 
+  if (G_UNLIKELY (!g_atomic_int_get (&self->active)))
+    return TRUE;
+
   self->output_flow = GST_FLOW_OK;
 
   gst_v4l2_object_unlock_stop (self->v4l2output);