Segfault with total neutron flux

[moatazharb]

meshtal2tk gives a segfault for total neutron flux results with mcnp6.1. MCNP gives the upper energy bound as 5.00E+01. The issue doesn't appear with a total photon flux in the same mesh tally file. When replacing the upper energy bound for neutrons from 5.00E+01 to 1.00E+36, that of the photons, the tool succeeds in generating the vts files. The lower energy bound doesn't seem to affect this, since the tested meshtal file had 1.00E-03 for photons and 1.00E-11 for neutrons.