avcodec/indeo2: check ctab

Fixes out of array access
Fixes: 6b73fa392ac808f02e95a4e0a5770026/asan_static-oob_1b15f9a_1969_e7778535e5f27225fe0d6ded14721430.AVI

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
