QPID-4462: simply return the PlainSaslServer directly instead of looking up it, bypassing the lack of previous registration

Fix suggested by Alex Rudyy

merged from trunk r1463063


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1463185 13f79535-47bb-0310-9956-ffa450edef68