distf integer overflow

[yslan]

This is a minor issue that won't change the result unless the overflow number get exact 0, nchange = 2^32 for example.

In distf (and also in cheap_dist?), we loop through all GLL points and go through their (3^ndim) neighbor so the potential number of changes can up to E * lx1^ndim * 3^ndim.

I have a case with nelgv = 10M, N=7 that print negative count

        1   -79624688  4.5219E-01 max wall distance 2
        2   -50506081  9.1576E-01 max wall distance 2
        3   -61069130  1.3114E+00 max wall distance 2
        4  -749316730  5.9702E-01 max wall distance 2
        5   743722746  5.3373E-01 max wall distance 2
        6   138545728  4.3998E-01 max wall distance 2
        7    37899754  4.2563E-01 max wall distance 2
        8     9461456  4.2106E-01 max wall distance 2
        9     4978723  4.1731E-01 max wall distance 2
       10     2482463  4.1665E-01 max wall distance 2
       11      806825  4.1586E-01 max wall distance 2
       12           0  0.0000E+00 max wall distance 2