lavc: make avcodec_decode_subtitle2 more robust

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -2338,6 +2338,12 @@ int avcodec_decode_subtitle2(AVCodecContext *avctx, AVSubtitle *sub,
 {
     int i, ret = 0;
 
+    if (!avpkt->data && avpkt->size) {
+        av_log(avctx, AV_LOG_ERROR, "invalid packet: NULL data, size != 0\n");
+        return AVERROR(EINVAL);
+    }
+    if (!avctx->codec)
+        return AVERROR(EINVAL);
     if (avctx->codec->type != AVMEDIA_TYPE_SUBTITLE) {
         av_log(avctx, AV_LOG_ERROR, "Invalid media type for subtitles\n");
         return AVERROR(EINVAL);