Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://issues.oss-fuzz.com/issues/42538163

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1923058 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFGraphicFrame.java

@@ -79,7 +79,7 @@ public class XSLFGraphicFrame extends XSLFShape implements GraphicalFrame<XSLFSh
         }
 
         CTPoint2D off = xfrm.getOff();
-        if (off == null) {
+        if (off == null || off.getX() == null || off.getY() == null) {
             throw new IllegalArgumentException("Could not retrieve Off from the XML object");
         }
         double x = Units.toPoints(POIXMLUnits.parseLength(off.xgetX()));