SMACK-438 Avoid NPE when the weak reference is null. Add InvitationsMonitor as strong reference within getInvitationsMonitor and return it within the block so it can't get gc'ed between put() and get().

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13648 b35dd754-fafc-0310-a699-88a17e54d16e
SMACK-439

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@13649 b35dd754-fafc-0310-a699-88a17e54d16e