libavfilter/vf_crop.c:308: error: Integer Overflow L2
  ([0, +oo] - 1):unsigned32 by call to `avfilter_unref_buffer`.

libavfilter/vf_crop.c:308:5: Call
306. 
307.     crop->var_values[VAR_N] += 1.0;
308.     avfilter_unref_buffer(link->cur_buf);
         ^
309.     avfilter_end_frame(link->dst->outputs[0]);
310. }

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);
