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

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 26b45096906097a73ba587bf3b98dada4e795224)
diff --git a/libavcodec/libvpxenc.c b/libavcodec/libvpxenc.c
index 4ea932d..542add4 100644
--- a/libavcodec/libvpxenc.c
+++ b/libavcodec/libvpxenc.c
@@ -805,7 +805,6 @@
                                                 cx_frame->sz_alpha + 8);
             if(!side_data) {
                 av_packet_unref(pkt);
-                av_free(pkt);
                 return AVERROR(ENOMEM);
             }
             AV_WB64(side_data, 1);