lavc: fix 10l oversight in realloc of avcodec_encode_video2.

Packets are not guranteed to be allocated by av_malloc().
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -1219,7 +1219,7 @@ int attribute_align_arg avcodec_encode_video2(AVCodecContext *avctx,
             avpkt->size = 0;
         else if (!(avctx->codec->capabilities & CODEC_CAP_DELAY))
             avpkt->pts = avpkt->dts = frame->pts;
-        if (avpkt->data) {
+        if (avpkt->data && avpkt->destruct == av_destruct_packet) {
             new_data = av_realloc(avpkt->data,
                                   avpkt->size + FF_INPUT_BUFFER_PADDING_SIZE);
             if (new_data)