Added additional check for negative values in URR probability tables

When running some test problems with the NNDC cross sections, I found several nuclides that have negative values in the URR probability tables:

 WARNING: Negative value(s) found on probability table for nuclide  11022.71c
 WARNING: Negative value(s) found on probability table for nuclide  18036.71c
 WARNING: Negative value(s) found on probability table for nuclide  34079.71c
 WARNING: Negative value(s) found on probability table for nuclide  41094.71c
 WARNING: Negative value(s) found on probability table for nuclide  41095.71c
 WARNING: Negative value(s) found on probability table for nuclide  42099.71c
 WARNING: Negative value(s) found on probability table for nuclide  48106.71c
 WARNING: Negative value(s) found on probability table for nuclide  50123.71c
 WARNING: Negative value(s) found on probability table for nuclide  53131.71c
 WARNING: Negative value(s) found on probability table for nuclide  55136.71c
 WARNING: Negative value(s) found on probability table for nuclide  63156.71c

This led to NaNs and an un-handled segfault.  This PR eliminates those NaNs at the expense of an additional comparison to make sure we're not taking the log of a negative number.