avcodec/aacenc: Check all coefficients for finiteness

This is needed as near infinite values on the input side result in only some
output to be non finite.
Also it may still be insufficient if subsequent computations overflow

Fixes null pointer dereference
Fixes: ae66c0f6c12ac1cd5c2c237031240f57/signal_sigsegv_2618c99_9516_6007026f2185a26d7afea895fbed6e38.ogg

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Reviewed-by: Claudio Freire <klaussfreire@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
