FindBugs fix
- fixed "equals() method does not check for null argument"
- see http://findbugs.sourceforge.net/bugDescriptions.html#NP_EQUALS_SHOULD_HANDLE_NULL_ARGUMENT

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568812 13f79535-47bb-0310-9956-ffa450edef68