Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=54438
Fix a regression in the fix for BZ 52953 that triggered an NPE when digested passwords were used and an authentication attempt was made for a user that did not exist in the realm.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1434685 13f79535-47bb-0310-9956-ffa450edef68