avcodec/aacsbr: check that the element type matches before applying SBR

Fixes out of array access
Fixes: signal_sigsegv_3670fc0_2818_cov_2307326154_moon.mux

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