matroskadec: use uint64_t instead of int for index_scale

index_scale is set to matroska->time_scale of type uint64_t.

When index_scale is int, the assignment can overflow and e.g. result
in index_scale = 0. This causes a floating point exception due to the
division by index_scale.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit eb9fb508b0)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -1441,7 +1441,7 @@ static void matroska_add_index_entries(MatroskaDemuxContext *matroska)
 {
     EbmlList *index_list;
     MatroskaIndex *index;
-    int index_scale = 1;
+    uint64_t index_scale = 1;
     int i, j;
 
     index_list = &matroska->index;