core: use config count not size for allocating space for options

`size` counts both actual options **and** hint entries within option
sets. `count` only counts the actual options. we are only interested in
the actual options here, so we should use `count` to allocate only the
memory we actually need. this avoids allocating roughly 1000 pointers'
worth of unused space.

the addition of the assert was requested in review.