Avoid segfault when nuclide is present in material that's not used
Avoid segfault when nuclide is present in material that's not used

wbinventor/openmc,walshjon/openmc,shikhar413/openmc,johnnyliu27/openmc,johnnyliu27/openmc,walshjon/openmc,paulromano/openmc,paulromano/openmc,walshjon/openmc,liangjg/openmc,wbinventor/openmc,mit-crpg/openmc,mit-crpg/openmc,amandalund/openmc,smharper/openmc,smharper/openmc,liangjg/openmc,paulromano/openmc,shikhar413/openmc,wbinventor/openmc,liangjg/openmc,walshjon/openmc,shikhar413/openmc,mit-crpg/openmc,johnnyliu27/openmc,smharper/openmc,paulromano/openmc,smharper/openmc,amandalund/openmc,johnnyliu27/openmc,mit-crpg/openmc,amandalund/openmc,amandalund/openmc,shikhar413/openmc,liangjg/openmc,wbinventor/openmc