avcodec/dnxhdenc: do not free nonexistent slice threads data when frame threading is used

Forgotten in 28e9ba95.
Fixes #7241.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/dnxhdenc.c

@@ -1369,8 +1369,10 @@ static av_cold int dnxhd_encode_end(AVCodecContext *avctx)
     av_freep(&ctx->qmatrix_c16);
     av_freep(&ctx->qmatrix_l16);
 
-    for (i = 1; i < avctx->thread_count; i++)
-        av_freep(&ctx->thread[i]);
+    if (avctx->active_thread_type == FF_THREAD_SLICE) {
+        for (i = 1; i < avctx->thread_count; i++)
+            av_freep(&ctx->thread[i]);
+    }
 
     return 0;
 }