Fixed PME bug with high OpenMP thread count

PME energies and forces could be incorrect with combined MPI+OpenMP
parallelization. This would, only, happen when
pmegrids->nthread_comm[YY] >= 2, which can only occur with high OpenMP
thread count with multiple large prime factors.
It's unlikely that this issue affected production runs.

Fixes #1572.

Change-Id: I03b38c279c8f8ab2e111dad0976edad88b3ea93b