Fixes #1625 issue by caching the field list and validation state in addition to the existing layoutInfo and fieldOrder caches.

Author: brettwooldridge

src/com/sun/jna/Structure.java

@@ -1018,10 +1018,10 @@ protected void sortFields(List<Field> fields, List<String> names) {
      */
     protected List<Field> getFieldList() {
         synchronized (fieldList) {
-            return fieldList.computeIfAbsent(getClass(), (c) -> {
+            return fieldList.computeIfAbsent(getClass(), (clazz) -> {
                 List<Field> flist = new ArrayList<>();
                 List<Field> classFields = new ArrayList<>();
-                for (Class<?> cls = getClass();
+                for (Class<?> cls = clazz;
                      !cls.equals(Structure.class);
                      cls = cls.getSuperclass()) {
                     for (Field field : cls.getDeclaredFields()) {