avformat/mxfdec: Fix DoS issues in mxf_read_index_entry_array()

      Fixes: 20170829A.mxf

Co-Author: 张洪亮(望初)" <wangchu.zhl@alibaba-inc.com>
Found-by: Xiaohei and Wangchu from Alibaba Security Team
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 900f39692ca0337a98a7cf047e4e2611071810c2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>