avcodec/aacenc: Check both channels for finiteness

Fixes null pointer dereference
Fixes: 10412fc52ecc6eab40ed67f82ca7b372/signal_sigsegv_2618c99_2129_f808373959e46afb165593332799ffbc.aif

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