indeo5: check tile size in decode_mb_info().

      This prevents writing into a too small array if some parameters changed
without the tile being reallocated.

Fixes CVE-2012-2794

CC:libav-stable@libav.org

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Anton Khirnov <anton@khirnov.net>