Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a few NullPointerException

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=66400

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1917068 13f79535-47bb-0310-9956-ffa450edef68

poi-examples/src/main/java/org/apache/poi/examples/xssf/eventusermodel/FromHowTo.java

@@ -136,7 +136,7 @@ public class FromHowTo {
             if(nextIsString && StringUtil.isNotBlank(lastContents)) {
                 Integer idx = Integer.valueOf(lastContents);
                 lastContents = lruCache.get(idx);
-                if (lastContents == null && !lruCache.containsKey(idx)) {
+                if (lastContents == null && !lruCache.containsKey(idx) && sst != null) {
                     lastContents = sst.getItemAt(idx).getString();
                     lruCache.put(idx, lastContents);
                 }