Added check for zero cross section when calculating distance to collision.
Added check for zero cross section when calculating distance to collision.

amandalund/openmc,shikhar413/openmc,samuelshaner/openmc,mit-crpg/openmc,keadyk/openmc_mg_prepush,bhermanmit/cdash,wbinventor/openmc,johnnyliu27/openmc,kellyrowland/openmc,liangjg/openmc,bhermanmit/openmc,samuelshaner/openmc,lilulu/openmc,shikhar413/openmc,shenqicang/openmc,johnnyliu27/openmc,smharper/openmc,liangjg/openmc,wbinventor/openmc,paulromano/openmc,smharper/openmc,paulromano/openmc,walshjon/openmc,smharper/openmc,bhermanmit/openmc,walshjon/openmc,shikhar413/openmc,walshjon/openmc,kellyrowland/openmc,mjlong/openmc,walshjon/openmc,lilulu/openmc,wbinventor/openmc,sxds/opemmc,nhorelik/openmc,wbinventor/openmc,samuelshaner/openmc,shenqicang/openmc,keadyk/openmc_mg_prepush,smharper/openmc,mit-crpg/openmc,shikhar413/openmc,liangjg/openmc,nhorelik/openmc,paulromano/openmc,amandalund/openmc,keadyk/openmc_mg_prepush,sxds/opemmc,amandalund/openmc,amandalund/openmc,lilulu/openmc,mjlong/openmc,johnnyliu27/openmc,johnnyliu27/openmc,mit-crpg/openmc,samuelshaner/openmc,mit-crpg/openmc,liangjg/openmc,paulromano/openmc