avcodec/libvpxdec: fix setting auto threads

a thread count of 0 is treated the same as 1, use av_cpu_count() to get
the correct thread count when auto threads is requested.

this matches the fix in libvpxenc:
27df34bf avcodec/libvpxenc: fix setting amount of threads used for encoding
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: James Zern <jzern@google.com>

libavcodec/libvpxdec.c

@@ -47,8 +47,7 @@ static av_cold int vpx_init(AVCodecContext *avctx,
 {
     VPxContext *ctx = avctx->priv_data;
     struct vpx_codec_dec_cfg deccfg = {
-        /* token partitions+1 would be a decent choice */
-        .threads = FFMIN(avctx->thread_count, 16)
+        .threads = FFMIN(avctx->thread_count ? avctx->thread_count : av_cpu_count(), 16)
     };
 
     av_log(avctx, AV_LOG_INFO, "%s\n", vpx_codec_version_str());