avcodec/avpacket: clear side_data_elems

Fixes null pointer dereference

Found-by: 连一汉 <lianyihan@360.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5e1bf9d8c0)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avpacket.c

@@ -195,6 +195,7 @@ static int copy_packet_data(AVPacket *pkt, const AVPacket *src, int dup)
 {
     pkt->data      = NULL;
     pkt->side_data = NULL;
+    pkt->side_data_elems = 0;
     if (pkt->buf) {
         AVBufferRef *ref = av_buffer_ref(src->buf);
         if (!ref)
@@ -209,9 +210,11 @@ FF_DISABLE_DEPRECATION_WARNINGS
     pkt->destruct = dummy_destruct_packet;
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
-    if (pkt->side_data_elems && dup)
+    if (src->side_data_elems && dup) {
         pkt->side_data = src->side_data;
-    if (pkt->side_data_elems && !dup) {
+        pkt->side_data_elems = src->side_data_elems;
+    }
+    if (src->side_data_elems && !dup) {
         return av_copy_packet_side_data(pkt, src);
     }
     return 0;