Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a few NullPointerException

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65450 and
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=63907 and
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=63727

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1915480 13f79535-47bb-0310-9956-ffa450edef68
3 files changed