mxfdec: check index_tables before dereferencing in close.

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mxfdec.c

@@ -2250,11 +2250,13 @@ static int mxf_read_close(AVFormatContext *s)
     av_freep(&mxf->aesc);
     av_freep(&mxf->local_tags);
 
+    if (mxf->index_tables) {
         for (i = 0; i < mxf->nb_index_tables; i++) {
             av_freep(&mxf->index_tables[i].segments);
             av_freep(&mxf->index_tables[i].ptses);
             av_freep(&mxf->index_tables[i].fake_index);
         }
+    }
     av_freep(&mxf->index_tables);
 
     return 0;