Fix bug when calculating index of OOP-bending parameters
Fix bug when calculating index of OOP-bending parameters

This fixes a minor bug when calculating the index for the
out of plane bending parameters in the MMFF force field.

Thanks to Robert Hanson for spotting this bug.

Change-Id: I9fac4fdf355fa3ea80381ee43c363c0da6e7f1da

kylelutz/chemkit,kylelutz/chemkit,kylelutz/chemkit,kylelutz/chemkit