Bug 66425: Avoid a ClassCastException found via oss-fuzz

We try to avoid throwing ClassCastException, but it was possible
to trigger one here with a specially crafted input-file

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912140 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -312,11 +312,11 @@ public class XWPFDocument extends POIXMLDocument implements Document, IBody {
         for (RelationPart rp : getRelationParts()) {
             POIXMLDocumentPart p = rp.getDocumentPart();
             String relation = rp.getRelationship().getRelationshipType();
-            if (relation.equals(XWPFRelation.FOOTNOTE.getRelation())) {
+            if (relation.equals(XWPFRelation.FOOTNOTE.getRelation()) && p instanceof XWPFFootnotes) {
                 this.footnotes = (XWPFFootnotes) p;
                 this.footnotes.onDocumentRead();
                 this.footnotes.setIdManager(footnoteIdManager);
-            } else if (relation.equals(XWPFRelation.ENDNOTE.getRelation())) {
+            } else if (relation.equals(XWPFRelation.ENDNOTE.getRelation()) && p instanceof XWPFEndnotes) {
                 this.endnotes = (XWPFEndnotes) p;
                 this.endnotes.onDocumentRead();
                 this.endnotes.setIdManager(footnoteIdManager);