lavc/libaomenc: Add a maximum constraint of 64 encoder threads.

fixed the error in Intel(R) Xeon(R) Gold 6152 CPU like:
[libaom-av1 @ 0x469f340] Failed to initialize encoder: Invalid parameter
[libaom-av1 @ 0x469f340]   Additional information: g_threads out of range [..MAX_NUM_THREADS]
Signed-off-by: Jun Zhao <mypopydev@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/libaomenc.c

@@ -504,7 +504,8 @@ static av_cold int aom_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 ? avctx->thread_count : av_cpu_count();
+    enccfg.g_threads      =
+        FFMIN(avctx->thread_count ? avctx->thread_count : av_cpu_count(), 64);
 
     if (ctx->lag_in_frames >= 0)
         enccfg.g_lag_in_frames = ctx->lag_in_frames;