libavfilter/vf_gradfun.c:230: error: Integer Overflow L2
  ([0, +oo] - 1):unsigned32 by call to `avfilter_unref_buffer`.

libavfilter/vf_gradfun.c:230:5: Call
228.     avfilter_draw_slice(outlink, 0, inlink->h, 1);
229.     avfilter_end_frame(outlink);
230.     avfilter_unref_buffer(inpic);
         ^
231.     avfilter_unref_buffer(outpic);
232. }

libavfilter/avfilter.c:71:1: <LHS trace>
69. }
70. 
71. void avfilter_unref_buffer(AVFilterBufferRef *ref)
    ^
72. {
73.     if (!(--ref->buf->refcount))

libavfilter/avfilter.c:71:1: Parameter `ref->buf->refcount`
  69. }
  70. 
  71. void avfilter_unref_buffer(AVFilterBufferRef *ref)
      ^
  72. {
  73.     if (!(--ref->buf->refcount))

libavfilter/avfilter.c:73:11: Binary operation: ([0, +oo] - 1):unsigned32 by call to `avfilter_unref_buffer` 
  71. void avfilter_unref_buffer(AVFilterBufferRef *ref)
  72. {
  73.     if (!(--ref->buf->refcount))
                ^
  74.         ref->buf->free(ref->buf);
  75.     av_free(ref->video);
