mxg: do not leak mxg->buffer

Signed-off-by: Paul B Mahol <onemda@gmail.com>
aacdec: dont pop random configurations

Fixes out of array accesses

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