Check that start_display_time is 0 in avcodec_encode_subtitle()

Patch by Björn Axelsson ( gecko A acc D umu D se )

Originally committed as revision 17092 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -501,6 +501,10 @@ int avcodec_encode_subtitle(AVCodecContext *avctx, uint8_t *buf, int buf_size,
                             const AVSubtitle *sub)
 {
     int ret;
+    if(sub->start_display_time) {
+        av_log(avctx, AV_LOG_ERROR, "start_display_time must be 0.\n");
+        return -1;
+    }
     ret = avctx->codec->encode(avctx, buf, buf_size, sub);
     avctx->frame_number++;
     return ret;