avcodec/libvpxenc: fix setting amount of threads used for encoding

The libvpx doxy says that a value of 0 for the g_threads field is
equivalent to a value of 1, whereas for avctx->thread_count it means
the maximum amount of threads possible for the host system.

Use av_cpu_count() to get the correct thread count when auto threads
is requested.
Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libvpxenc.c

@@ -493,7 +493,7 @@ static av_cold int vpx_init(AVCodecContext *avctx,
     enccfg.g_h            = avctx->height;
     enccfg.g_timebase.num = avctx->time_base.num;
     enccfg.g_timebase.den = avctx->time_base.den;
-    enccfg.g_threads      = avctx->thread_count;
+    enccfg.g_threads      = avctx->thread_count ? avctx->thread_count : av_cpu_count();
     enccfg.g_lag_in_frames= ctx->lag_in_frames;
 
     if (avctx->flags & AV_CODEC_FLAG_PASS1)