indeo4/5: check empty tile size in decode_mb_info().

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

Based on a patch by Michael Niedermayer <michaelni@gmx.at>

Fixes CVE-2012-2800

CC:libav-stable@libav.org

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
