bbox_collision_detector: constify local vars; fix a bug with NumDimensions==0
bbox_collision_detector: constify local vars; fix a bug with NumDimensions==0

Adding max_in_array_of_unsigned() helped both to constify and bugfix that.
It's an "undefined behaviour" bug in a generic library, so
worth the silliness of supporting zero dimensions where it's easy.
Given optimizations, there's no runtime cost here.

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