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

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912139 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/ooxml/POIXMLRelation.java

@@ -205,6 +205,9 @@ public abstract class POIXMLRelation {
             PackageRelationship rel = it.next();
             PackagePartName relName = PackagingURIHelper.createPartName(rel.getTargetURI());
             PackagePart part = corePart.getPackage().getPart(relName);
+            if (part == null) {
+                throw new IllegalArgumentException("Could not read part " + relName + " from " + corePart);
+            }
             return part.getInputStream();
         }
         LOGGER.atWarn().log("No part {} found", getDefaultFileName());