avcodec/ffv1dec: Clear slice coordinates if they are invalid or slice header decoding fails for other reasons

Fixes Ticket4931

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

libavcodec/ffv1dec.c

@@ -408,6 +408,7 @@ static int decode_slice(AVCodecContext *c, void *arg)
         if (ff_ffv1_init_slice_state(f, fs) < 0)
             return AVERROR(ENOMEM);
         if (decode_slice_header(f, fs) < 0) {
+            fs->slice_x = fs->slice_y = fs->slice_height = fs->slice_width = 0;
             fs->slice_damaged = 1;
             return AVERROR_INVALIDDATA;
         }