[bugfix] Prevent out-of-bounds indexing during pinch processing.

[blattms]

If we are processing a cell at the bottom of the grid, then there will be no lower neighbor at all and certainly not for an NNC.

Previously we would compute the index of a cell in the next non-existing layer and sometimes would index containers actnum and thickness out of bounds.

[blattms]

Should make it to the release.

[blattms]

jenkins build this please