lavf/mov: don't read outside frag_index bounds

Potentially fixes:
https://bugs.chromium.org/p/chromium/issues/detail?id=786269#c1

In theory, the crash can be triggered by an invalid stream that has
either tfdt or trun outside of the moof
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -1188,6 +1188,10 @@ static void set_frag_stream(MOVFragmentIndex *frag_index, int id)
 static MOVFragmentStreamInfo * get_current_frag_stream_info(
     MOVFragmentIndex *frag_index)
 {
+    if (frag_index->current < 0 ||
+        frag_index->current >= frag_index->nb_items)
+        return NULL;
+
     MOVFragmentIndexItem * item = &frag_index->item[frag_index->current];
     if (item->current >= 0 && item->current < item->nb_stream_info)
         return &item->stream_info[item->current];