When the key is null (i.e., a stale entry), it cannot be removed with an explicit...

When the key is null (i.e., a stale entry), it cannot be removed with an explicit remove(key) call: you'll get an NPE. Those can be removed with expungeStaleEntries() call.
Also, simplified the code: Reference.referent can be accessed by calling get() - no need to use reflection for that.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@909212 13f79535-47bb-0310-9956-ffa450edef68