Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=66386

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1917069 13f79535-47bb-0310-9956-ffa450edef68

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

             for (int i = 0; i < nodes.getLength(); i++) {
                 final Node node = nodes.item(i);
                 // if the frame references a chart, associate the chart with this instance
-                if (node.getNodeName().equals("c:chart")) {
+                Node namedItem = node.getAttributes().getNamedItem("r:id");
+                if (node.getNodeName().equals("c:chart") && namedItem != null) {
                     // this better succeed or the document is invalid
-                    POIXMLDocumentPart relation = drawing.getRelationById(node.getAttributes().getNamedItem("r:id").getNodeValue());
+                    POIXMLDocumentPart relation = drawing.getRelationById(namedItem.getNodeValue());
                     // Do XWPF charts need similar treatment?
                     if (relation instanceof XSSFChart) {
                         ((XSSFChart) relation).setGraphicFrame(this);