valid: Allow xmlFreeValidCtxt(NULL)

valid.c

@@ -879,6 +879,8 @@ xmlValidCtxtPtr xmlNewValidCtxt(void) {
  */
 void
 xmlFreeValidCtxt(xmlValidCtxtPtr cur) {
+    if (cur == NULL)
+        return;
     if (cur->vstateTab != NULL)
         xmlFree(cur->vstateTab);
     if (cur->nodeTab != NULL)