Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65448

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1917175 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xwpf/usermodel/XWPFFootnotes.java

@@ -127,10 +127,13 @@ public class XWPFFootnotes extends XWPFAbstractFootnotesEndnotes {
         XmlOptions xmlOptions = new XmlOptions(DEFAULT_XML_OPTIONS);
         xmlOptions.setSaveSyntheticDocumentElement(new QName(CTFootnotes.type.getName().getNamespaceURI(), "footnotes"));
         PackagePart part = getPackagePart();
+
+        if (ctFootnotes != null) {
             try (OutputStream out = part.getOutputStream()) {
                 ctFootnotes.save(out, xmlOptions);
             }
         }
+    }
 
     /**
      * Add an {@link XWPFFootnote} to the document