Bug 66425: Avoid NullPointerExceptions found via poi-fuzz

We try to avoid throwing NullPointerException, but it was possible
to trigger one here

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912303 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -1430,8 +1430,9 @@ private void _getText(XmlObject o, StringBuilder text) {
         }
         if (o instanceof CTFtnEdnRef) {
             CTFtnEdnRef ftn = (CTFtnEdnRef) o;
+            final int i = ftn.getId() == null ? -1 : ftn.getId().intValue();
             String footnoteRef = ftn.getDomNode().getLocalName().equals("footnoteReference") ?
-                    "[footnoteRef:" + ftn.getId().intValue() + "]" : "[endnoteRef:" + ftn.getId().intValue() + "]";
+                    "[footnoteRef:" + i + "]" : "[endnoteRef:" + i + "]";
             text.append(footnoteRef);
         }
     }