We try to avoid throwing NullPointerException, but it was possible
to trigger one here with a specially crafted input-file

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=61644

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1911842 13f79535-47bb-0310-9956-ffa450edef68