Bug 66425: Avoid Exceptions found via oss-fuzz

We try to avoid throwing NullPointerExceptions or endless allocations,
but it was possible to trigger one here with a specially
crafted input-file

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=62745

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912795 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xdgf/usermodel/section/geometry/LineTo.java

         for (CellType cell : row.getCellArray()) {
             String cellName = cell.getN();
 
-            if (cellName.equals("X")) {
+            if ("X".equals(cellName)) {
                 x = XDGFCell.parseDoubleValue(cell);
-            } else if (cellName.equals("Y")) {
+            } else if ("Y".equals(cellName)) {
                 y = XDGFCell.parseDoubleValue(cell);
             } else {
                 throw new POIXMLException("Invalid cell '" + cellName