Fix a hang due to a race condition when processes call MPI_Abort.

If a process was being forcibly killed instead of reaching the call to "abort", then we would accidentally increment a counter in one place while decrementing a counter in another - thus putting them off when comparing the two. This ensures we only increment the one side, and leave the other alone.

No corresponding master commit as this problem doesn't exist there.

@ggouaillardet please review