avcodec/vc2enc: Check for non negative slice bounds

Fixes: invalid shifts
Fixes: Ticket 8221

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vc2enc.c

@@ -982,6 +982,8 @@ static av_cold int vc2_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     }
 
     s->slice_min_bytes = s->slice_max_bytes - s->slice_max_bytes*(s->tolerance/100.0f);
+    if (s->slice_min_bytes < 0)
+        return AVERROR(EINVAL);
 
     ret = encode_frame(s, avpkt, frame, aux_data, header_size, s->interlaced);
     if (ret)