FindBugs fix
- fixed/checked various null pointer related entries
- see http://findbugs.sourceforge.net/bugDescriptions.html#NP_NULL_PARAM_DEREF
- ... NP_NULL_ON_SOME_PATH, NP_NULL_ON_SOME_PATH_EXCEPTION


git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1568789 13f79535-47bb-0310-9956-ffa450edef68