lavc/avpacket: Fix undefined behaviour, do not pass a null pointer to memcpy().

Fixes ticket #5857.
(cherry picked from commit c54eef46)

libavcodec/avpacket.c

@@ -139,6 +139,7 @@ int av_grow_packet(AVPacket *pkt, int grow_by)
         pkt->buf = av_buffer_alloc(new_size);
         if (!pkt->buf)
             return AVERROR(ENOMEM);
+        if (pkt->size > 0)
             memcpy(pkt->buf->data, pkt->data, pkt->size);
         pkt->data = pkt->buf->data;
     }