Bug 66425: Avoid exceptions found via poi-fuzz

Prevent a NullPointerException

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=70273

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1919213 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xdgf/usermodel/XDGFSheet.java

@@ -73,9 +73,9 @@ public abstract class XDGFSheet {
 
             for (SectionType section: sheet.getSectionArray()) {
                 String name = section.getN();
-                if (name.equals("Geometry")) {
+                if ("Geometry".equals(name)) {
                     _geometry.put(section.getIX(), new GeometrySection(section, this));
-                } else if (name.equals("Character")) {
+                } else if ("Character".equals(name)) {
                     _character = new CharacterSection(section, this);
                 } else {
                     _sections.put(name, XDGFSection.load(section, this));