libavcodec/libvpxenc: Don't free user-provided AVPacket

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/libvpxenc.c

@@ -1148,7 +1148,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
                                                 cx_frame->sz_alpha + 8);
             if(!side_data) {
                 av_packet_unref(pkt);
-                av_free(pkt);
                 return AVERROR(ENOMEM);
             }
             AV_WB64(side_data, 1);