Fix logic in init_ewald_coulomb_force_table

Fix logic in init_ewald_coulomb_force_table

Previously, destroyParamLookupTable was called only if buffer was
nullptr (uninitialized). Which looks like a typo, and leads to nothing,
because all implementation of destroyParamLookupTable would refuse to
work on nullptr. Changed the check to free the buffer only if there is
anything to free.