Fix bug in stochastic volume calculation with void cell
Fix bug in stochastic volume calculation with void cell

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