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=61644

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

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

@@ -57,8 +57,10 @@ public abstract class XWPFAbstractFootnotesEndnotes extends POIXMLDocumentPart {
 
     public XWPFAbstractFootnoteEndnote getFootnoteById(int id) {
         for (XWPFAbstractFootnoteEndnote note : listFootnote) {
-            if (note.getCTFtnEdn().getId().intValue() == id)
+            if (note.getCTFtnEdn().getId() != null &&
+                    note.getCTFtnEdn().getId().intValue() == id) {
                 return note;
+            }
         }
         return null;
     }