Avoid NaN in rlist buffer calculation

With constraints on large masses and very low tolerance, the Verlet
buffer calculation could produce 1/0. This could lead to a somewhat
too smaller buffer.
Also added a missing scaling factor to the contribution of the third
derivative of the potential. This issue only caused a minor
overestimate for systems with constraints and without electrostatics.

Change-Id: I97e9d428a83f1b4954012ebd39bc49d397574f8c