avcodec/decode: Return EAGAIN instead of discarding unused packet

Should fix #10457, a regression caused by
69516ab3e9.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/decode.c

@@ -666,7 +666,9 @@ int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacke
     if (avpkt && !avpkt->size && avpkt->data)
         return AVERROR(EINVAL);
 
-    av_packet_unref(avci->buffer_pkt);
+    if (!AVPACKET_IS_EMPTY(avci->buffer_pkt))
+        return AVERROR(EAGAIN);
+
     if (avpkt && (avpkt->data || avpkt->side_data_elems)) {
         ret = av_packet_ref(avci->buffer_pkt, avpkt);
         if (ret < 0)