LOG4J-462, LOG4J-465 Log4jLogEvent constructor and RingBufferLogEvent#getLevel convert null Level to Level.OFF to prevent NPEs.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1555339 13f79535-47bb-0310-9956-ffa450edef68