`BoundingBox` constructor doesn't work when `n <= 0`

[ebchin]

There should be a return after this line:

https://github.com/LLNL/axom/blob/db9d01d6eef190bccb23cf4e609f2d7b729973b3/src/axom/primal/geometry/BoundingBox.hpp#L411

Otherwise L416 sets m_min and m_max to an invalid address.

[kennyweiss]

Thanks @ebchin!

I'll attach this to a small bugfix PR that I just opened -- #1603