vss: Cleanup num_slices().

vss: Cleanup num_slices().

This makes num_slices() take ints rather than uint8_t arguments which
avoids nasty integer overflows and allows to drop the result parameter
since the result is now contained in the return value.

Moreover, num_slices() now takes the number of redundant slices as
an additional argument, so that this value has to be comuted only
once in the caller.