Fix crash when running with --audit.
Fix crash when running with --audit.

When running with --audit, if an anonymous feature is present, VW will segv.

hal3/vowpal_wabbit,michaelbrundage/vowpal_wabbit,pombredanne/vowpal_wabbit,pombredanne/vowpal_wabbit,trufanov-nok/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,michaelbrundage/vowpal_wabbit,michaelbrundage/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,danmelamed/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,hal3/vowpal_wabbit,lhoang29/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,pombredanne/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,trufanov-nok/vowpal_wabbit,hal3/vowpal_wabbit,pombredanne/vowpal_wabbit,hal3/vowpal_wabbit,pombredanne/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lhoang29/vowpal_wabbit,deaktator/vowpal_wabbit,trufanov-nok/vowpal_wabbit,lmatthieu/myvw,pombredanne/vowpal_wabbit,lhoang29/vowpal_wabbit,deaktator/vowpal_wabbit,pombredanne/vowpal_wabbit,lhoang29/vowpal_wabbit,michaelbrundage/vowpal_wabbit,lhoang29/vowpal_wabbit,deaktator/vowpal_wabbit,lhoang29/vowpal_wabbit,lmatthieu/myvw,danmelamed/vowpal_wabbit,lmatthieu/myvw,hal3/vowpal_wabbit,danmelamed/vowpal_wabbit,danmelamed/vowpal_wabbit,deaktator/vowpal_wabbit,danmelamed/vowpal_wabbit,hal3/vowpal_wabbit,michaelbrundage/vowpal_wabbit,danmelamed/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,lhoang29/vowpal_wabbit,hal3/vowpal_wabbit,deaktator/vowpal_wabbit,trufanov-nok/vowpal_wabbit,danmelamed/vowpal_wabbit,michaelbrundage/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,michaelbrundage/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,hal3/vowpal_wabbit,lhoang29/vowpal_wabbit,michaelbrundage/vowpal_wabbit,pombredanne/vowpal_wabbit,danmelamed/vowpal_wabbit,michaelbrundage/vowpal_wabbit,lhoang29/vowpal_wabbit,hal3/vowpal_wabbit,lhoang29/vowpal_wabbit,trufanov-nok/vowpal_wabbit,hal3/vowpal_wabbit,trufanov-nok/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,trufanov-nok/vowpal_wabbit,trufanov-nok/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,danmelamed/vowpal_wabbit,deaktator/vowpal_wabbit,hal3/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,danmelamed/vowpal_wabbit,lmatthieu/myvw,trufanov-nok/vowpal_wabbit,jon-morra-zefr/vowpal_wabbit,deaktator/vowpal_wabbit,lmatthieu/myvw,pombredanne/vowpal_wabbit,michaelbrundage/vowpal_wabbit,pombredanne/vowpal_wabbit,trufanov-nok/vowpal_wabbit