avformat_new_stream: fix initial last_dts value

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2869,6 +2869,7 @@ AVStream *avformat_new_stream(AVFormatContext *s, AVCodec *c)
         av_free(st);
         return NULL;
     }
+    st->info->last_dts = AV_NOPTS_VALUE;
 
     st->codec = avcodec_alloc_context3(c);
     if (s->iformat) {