avcodec/ffv1enc: allocate padding for extradata

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ffv1enc.c

@@ -549,7 +549,7 @@ static int write_extradata(FFV1Context *f)
 
     f->avctx->extradata_size = 10000 + 4 +
                                     (11 * 11 * 5 * 5 * 5 + 11 * 11 * 11) * 32;
-    f->avctx->extradata = av_malloc(f->avctx->extradata_size);
+    f->avctx->extradata = av_malloc(f->avctx->extradata_size + FF_INPUT_BUFFER_PADDING_SIZE);
     if (!f->avctx->extradata)
         return AVERROR(ENOMEM);
     ff_init_range_encoder(c, f->avctx->extradata, f->avctx->extradata_size);