avcodec/dcadec: remove fishy FFMAX()

These where intended to maintain the previous behavior before dca_dmix_code()
but it is unclear (to me) which way is correct and no sample seem to trigger
the case, also they are incomplete for the purprose of error checking

Found-by: Niels Möller <nisse@lysator.liu.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/dcadec: Check dca_dmixtable index

Found-by: Niels Möller <nisse@lysator.liu.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>