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

Fixes https://oss-fuzz.com/testcase-detail/5265527465181184

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912277 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xssf/usermodel/XSSFGraphicFrame.java

@@ -54,8 +54,10 @@ public final class XSSFGraphicFrame extends XSSFShape {
     protected XSSFGraphicFrame(XSSFDrawing drawing, CTGraphicalObjectFrame ctGraphicFrame) {
         this.drawing = drawing; // protected field on XSSFShape
         this.graphicFrame = ctGraphicFrame;
+
         // TODO: there may be a better way to delegate this
-        CTGraphicalObjectData graphicData = graphicFrame.getGraphic().getGraphicData();
+        CTGraphicalObjectData graphicData = graphicFrame.getGraphic() == null ?
+                null : graphicFrame.getGraphic().getGraphicData();
         if (graphicData != null) {
             NodeList nodes = graphicData.getDomNode().getChildNodes();
             for (int i = 0; i < nodes.getLength(); i++) {