avformat/mov: fix crash when trying to get a fragment time for a non-existing fragment

Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Marth64 <marth64@proxyid.net>
Signed-off-by: Marth64 <marth64@proxyid.net>
(cherry picked from commit 7c9bde1d0d)

libavformat/mov.c

@@ -1644,6 +1644,8 @@ static int64_t get_frag_time(AVFormatContext *s, AVStream *dst_st,
     // to fragments that referenced this stream in the sidx
     if (sc->has_sidx) {
         frag_stream_info = get_frag_stream_info(frag_index, index, sc->id);
+        if (!frag_stream_info)
+            return AV_NOPTS_VALUE;
         if (frag_stream_info->sidx_pts != AV_NOPTS_VALUE)
             return frag_stream_info->sidx_pts;
         if (frag_stream_info->first_tfra_pts != AV_NOPTS_VALUE)