A small bugfix enabling half-bounded optimisation to work as intended.

This fixes an obscure but simple bug, it is very unlikely anyone else has ever noticed it or ever would, but it is a bug and I can't knowingly leave it unfixed.  It is only encountered when using cogent.optimisers directly to minimise/maximise a function with the input(s) bound on only one side, eg: minimise(f, x, bounds=(0, None)).