uneven_block distribution does not handle empty processes correctly

When creating an instance of uneven_block with the distribution [1, 0, 1] then the implementation incorrectly returns process id 1 (counting from 0) to be the owner of element id 1 (counting from zero), because the local starts of processes 1 and 2 are identical (local starts would be [0, 1, 1, 2] in this case).

In general, the implementation incorrectly assigns elements belonging to the first owner after one or more adjacent empty owners to the first of those empty ones.

I would suggest to change the implementation from
https://github.com/boostorg/graph_parallel/blob/22385ccb6c35fcb07971f295807519bd031ed57d/include/boost/graph/parallel/distribution.hpp#L225-L226
to
```
size_vector::const_iterator ub = std::upper_bound(local_starts.begin(), local_starts.end(), (std::size_t) i);
return (--ub) - local_starts.begin();
```

Edit: Corrected mistake as pointed out below.