pathplan Pobsopen: use a 'size_t' when counting objects

pathplan Pobsopen: use a 'size_t' when counting objects

This squashes 3 -Wsign-conversion warnings and is generally closer to what we
would like to do here. The “proper” fix is for fields like `vconfig_t.N` to
become `size_t` instead of `int`. But unfortunately they are part of the public
API, and it seems undesirable to break API for this.

Note that an assumption previously implicit in this function, that all inputs
had a non-negative polygon count, is now an explicit assertion.