avcodec/libxvid: remove unnecessary output packet data check

The user buffers passed to avcodec_encode_video2() haven't been propagated to
AVCodec.encode2 implementations since 93016f5d1d.
Also, the generic encode code already unrefs the packet if nothing was encoded.

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

libavcodec/libxvid.c

@@ -738,7 +738,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
 static int xvid_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
                              const AVFrame *picture, int *got_packet)
 {
-    int xerr, i, ret, user_packet = !!pkt->data;
+    int xerr, i, ret;
     struct xvid_context *x = avctx->priv_data;
     int mb_width  = (avctx->width  + 15) / 16;
     int mb_height = (avctx->height + 15) / 16;
@@ -866,8 +866,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
 
         return 0;
     } else {
-        if (!user_packet)
-            av_packet_unref(pkt);
         if (!xerr)
             return 0;
         av_log(avctx, AV_LOG_ERROR,