demux/packet_pool: always return default packet state from pop

This is not functional change, as pop() is only used in packet_create()
which already cleared the packet, but it's better to make it explcit.

demux/packet.c

@@ -85,13 +85,13 @@ void demux_packet_unref(struct demux_packet *dp)
 static struct demux_packet *packet_create(struct demux_packet_pool *pool)
 {
     struct demux_packet *dp = pool ? demux_packet_pool_pop(pool) : NULL;
-    struct AVPacket *avpkt = dp ? dp->avpacket : NULL;
     if (!dp) {
         dp = talloc(NULL, struct demux_packet);
         talloc_set_destructor(dp, packet_destroy);
+        set_packet_defaults(dp);
     }
-    set_packet_defaults(dp);
-    dp->avpacket = avpkt ? avpkt : av_packet_alloc();
+    if (!dp->avpacket)
+        dp->avpacket = av_packet_alloc();
     MP_HANDLE_OOM(dp->avpacket);
     return dp;
 }

demux/packet_pool.c

@@ -120,11 +120,7 @@ struct demux_packet *demux_packet_pool_pop(struct demux_packet_pool *pool)
     // a single packet at a time. This avoids the need to clear potentially
     // hundreds of thousands of packets at once when file playback is stopped,
     // which would require a significant amount of time to iterate over all packets.
-    if (dp) {
-        if (dp->avpacket)
-            av_packet_unref(dp->avpacket);
-        ta_free_children(dp);
-    }
+    demux_packet_unref(dp);
 
     return dp;
 }

demux/packet_pool.h

@@ -82,6 +82,9 @@ void demux_packet_pool_prepend(struct demux_packet_pool *pool,
  * This function removes and returns the first packet from the pool's
  * linked list. This function is thread-safe.
  *
+ * The packet's contents are cleared of any external references before
+ * being returned.
+ *
  * @param pool Pointer to the demux packet pool.
  * @return Pointer to the demux packet, or NULL if the pool is empty.
  */