avcodec/vp9: assert on sane size in update_size().

w and h are both read as uint16 + 1 so this can not happen. A similar
change was introduced in 97962b27 / 72ca830f, with the
av_log()+AVERROR_INVALIDDATA form, suggesting it could be triggerable
somehow.

Change suggested by Ronald S. Bultje.

libavcodec/vp9.c

@@ -243,6 +243,8 @@ static int update_size(AVCodecContext *ctx, int w, int h)
     VP9Context *s = ctx->priv_data;
     uint8_t *p;
 
+    av_assert0(w > 0 && h > 0);
+
     if (s->above_partition_ctx && w == ctx->width && h == ctx->height)
         return 0;