lavc/decode: do not perform decoding when sending draining packets

This way decoding errors will not be returned when the user starts
draining the decoder, avoiding confusion over whether draining did or
did not start.

Fixes failures of fate-h264-attachment-631 for certain numbers of frame
threads (e.g. 5).

libavcodec/decode.c

@@ -674,7 +674,7 @@ int attribute_align_arg avcodec_send_packet(AVCodecContext *avctx, const AVPacke
     } else
         dc->draining_started = 1;
 
-    if (!avci->buffer_frame->buf[0]) {
+    if (!avci->buffer_frame->buf[0] && !dc->draining_started) {
         ret = decode_receive_frame_internal(avctx, avci->buffer_frame);
         if (ret < 0 && ret != AVERROR(EAGAIN) && ret != AVERROR_EOF)
             return ret;