Bug 66425: Avoid a NullPointerException found via oss-fuzz

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=62208

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912251 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xssf/eventusermodel/ReadOnlySharedStringsTable.java

         } else if ("rPh".equals(localName)) {
             inRPh = true;
             //append space...this assumes that rPh always comes after regular <t>
-            if (includePhoneticRuns && characters.length() > 0) {
+            if (includePhoneticRuns && characters != null && characters.length() > 0) {
                 characters.append(" ");
             }
         }
     public void characters(char[] ch, int start, int length) throws SAXException {
         if (tIsOpen) {
             if (inRPh && includePhoneticRuns) {
-                characters.append(ch, start, length);
+                if (characters != null) {
+                    characters.append(ch, start, length);
+                }
             } else if (! inRPh){
                 if (characters != null) {
                     characters.append(ch, start, length);