mlvdec: check that index_entries exist

This fixes NULL pointer dereferencing.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/mlvdec.c

@@ -365,6 +365,11 @@ static int read_header(AVFormatContext *avctx)
     if (ast)
         ast->duration = ast->nb_index_entries;
 
+    if ((vst && !vst->nb_index_entries) || (ast && !ast->nb_index_entries)) {
+        av_log(avctx, AV_LOG_ERROR, "no index entries found\n");
+        return AVERROR_INVALIDDATA;
+    }
+
     if (vst && ast)
         avio_seek(pb, FFMIN(vst->index_entries[0].pos, ast->index_entries[0].pos), SEEK_SET);
     else if (vst)