Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=55684
Log a warning but continue if the memory leak detection code is unable to access all threads.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1536634 13f79535-47bb-0310-9956-ffa450edef68