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

Fixes https://oss-fuzz.com/testcase-detail/4959857092198400

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912157 13f79535-47bb-0310-9956-ffa450edef68

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

@@ -71,11 +71,11 @@ public abstract class XWPFHeaderFooter extends POIXMLDocumentPart implements IBo
      */
     public XWPFHeaderFooter(POIXMLDocumentPart parent, PackagePart part) {
         super(parent, part);
-        this.document = (XWPFDocument) getParent();
-
-        if (this.document == null) {
-            throw new NullPointerException();
+        final POIXMLDocumentPart p = getParent();
+        if (!(p instanceof XWPFDocument)) {
+            throw new IllegalArgumentException("Had unexpected type of parent: " + (p == null ? "<null>" : p.getClass()));
         }
+        this.document = (XWPFDocument) p;
     }
 
     @Override

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

@@ -271,4 +271,12 @@ public final class XWPFRelation extends POIXMLRelation {
         return _table.get(rel);
     }
 
+    @Override
+    public String toString() {
+        return "XWPFRelation{" +
+                //getRelation() + "/" +
+                getContentType() + "/" +
+                getDefaultFileName() +
+                "}";
+    }
 }