Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=57208
Fixed NPE in JNDIRealm when "getUserBySearch" is performed, and no results are found. Based on a patch made by Jason McIntosh.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1639653 13f79535-47bb-0310-9956-ffa450edef68