avcodec/adpcm: Check idelta

Fixes integer overflow
Fixes: signal_sigsegv_1b0a4da_1865_cov_2167818389_computer_anger.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>