movenc: Check that frag_info entries exist in mov_write_sidx_tag

This fixes crashes with pathological cases when trying to write
a sidx index (via the -movflags faststart option, in combination
with fragmenting options), when no fragments actually have been
written. (This is possible if the empty_moov flag isn't used,
so that all actual packet data is written in the moov/mdat pair,
and no moof/mdat pairs have been written.)

In these pathological cases, no sidx should be written at all.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -2751,6 +2751,8 @@ static int mov_write_sidx_tag(AVIOContext *pb,
         }
     } else {
         entries = track->nb_frag_info;
+        if (entries <= 0)
+            return 0;
         presentation_time = track->frag_info[0].time;
     }