sndio: use sio_nfds
sndio: use sio_nfds

Instead of guessing the number of needed pollfd structures, use sio_nfds
and allocate the number of required structures.
If libsndio uses its alsa backend then in my tests the number of
required pollfds is 16, larger than the previously defined MAXFDs.
This resulted in sio_pollfd overwriting stack memory.

kinetiknz/cubeb,kinetiknz/cubeb,kinetiknz/cubeb