Fixed xref_current_source for sources that have no modification info
Fixed xref_current_source for sources that have no modification info

jn7163/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,jn7163/swipl-devel,edechter/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,jn7163/swipl-devel,mndrix/swipl-devel,koryonik/swipl-devel,mndrix/swipl-devel,edechter/swipl-devel