avcodec/dcadec: Check that the added xch channel isnt already there

Fixes null pointer dereference
Fixes: signal_sigsegv_369609d_623_cov_2008234281_ES_6.1_16bit.dts
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
