label overflow in checkMesh

In checkMesh: https://develop.openfoam.com/Development/OpenFOAM-plus/blob/develop/applications/utilities/mesh/manipulation/checkMesh/checkTools.C#L107

I don't see why we have scalar(nFaces + nIntFaces) instead of just scalar(nFaces). However, it this is intended, it should probably be formulated as (scalar(nFaces) + scalar(nIntFaces)) to avoid label overflow.

partial reference from EP#482

@Prashant