fixed bug that could produce wrong counter values

Bug:
- sometimes, addChunkHierarchy would return NULL (probably race-condition)
- in these cases, `_ptes[page].count` would remain too high
- as a result, memory is 'lost'


This bugfix was backported from https://github.com/ComputationalRadiationPhysics/mallocMC/commit/e66205cb56c362f69d09b6f4a4ccec3285c12787