Added correction for case when cluster is the size of the whole tract

An error is generated when a cluster is the size of the whole tract for one of the permutation `ii`. This is because in this case `clusEnd = find(pThresh(ii,:) == 0)` and thus `clusSiz = diff(clusEnd)` are empty  and therefore `max(clusSiz)` generates an error.

A possible correction is to set the maximum cluster size to the size of the tract in such a case:
```clusMax(ii) = size(pThresh(ii,:),2)```