avcodec/nellymoser: Use avpriv_float_dsp_alloc()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_amix: Use avpriv_float_dsp_alloc()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/af_volume: Use avpriv_float_dsp_alloc()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>