Avoid NPE if a signed Assertion does not have a KeyInfo Element

git-svn-id: https://svn.apache.org/repos/asf/webservices/wss4j/branches/1_6_x-fixes@1546009 13f79535-47bb-0310-9956-ffa450edef68
Updating backmerge

git-svn-id: https://svn.apache.org/repos/asf/webservices/wss4j/branches/1_6_x-fixes@1546010 13f79535-47bb-0310-9956-ffa450edef68