avcodec/aacdec: Fix pulse position checks in decode_pulses()

Fixes out of array read
Fixes: asan_static-oob_1efed25_1887_cov_2013541199_HeyYa_RA10_AAC_192K_30s.rm
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
