avcodec/pthread_frame.c: keep the last_pkt_props from worker threads in sync with the user context

Making it point to the input packet results in different behavior during flush,
where its contents will be that of an empty packet instead of containing the
props from the last input packet fed to the decoder.
After this change, decoding with more than one thread will shield the same
results as using a single thread.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/pthread_frame.c

@@ -371,6 +371,8 @@ FF_ENABLE_DEPRECATION_WARNINGS
  */
 static int update_context_from_user(AVCodecContext *dst, AVCodecContext *src)
 {
+    int err;
+
     dst->flags          = src->flags;
 
     dst->draw_horiz_band= src->draw_horiz_band;
@@ -406,6 +408,12 @@ FF_ENABLE_DEPRECATION_WARNINGS
                src->slice_count * sizeof(*dst->slice_offset));
     }
     dst->slice_count = src->slice_count;
+
+    av_packet_unref(dst->internal->last_pkt_props);
+    err = av_packet_copy_props(dst->internal->last_pkt_props, src->internal->last_pkt_props);
+    if (err < 0)
+        return err;
+
     return 0;
 }
 
@@ -775,6 +783,7 @@ void ff_frame_thread_free(AVCodecContext *avctx, int thread_count)
             av_freep(&ctx->slice_offset);
 
             av_buffer_unref(&ctx->internal->pool);
+            av_packet_free(&ctx->internal->last_pkt_props);
             av_freep(&ctx->internal);
             av_buffer_unref(&ctx->hw_frames_ctx);
         }
@@ -848,11 +857,14 @@ static av_cold int init_thread(PerThreadContext *p, int *threads_to_free,
     if (!(p->frame = av_frame_alloc()) ||
         !(p->avpkt = av_packet_alloc()))
         return AVERROR(ENOMEM);
-    copy->internal->last_pkt_props = p->avpkt;
 
     if (!first)
         copy->internal->is_copy = 1;
 
+    copy->internal->last_pkt_props = av_packet_alloc();
+    if (!copy->internal->last_pkt_props)
+        return AVERROR(ENOMEM);
+
     if (codec->init) {
         err = codec->init(copy);
         if (err < 0) {