Clean up XFA code which causes warnings

xfa/fxfa/parser/xfa_utils.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef XFA_FXFA_PARSER_XFA_UTILS_H_
 #define XFA_FXFA_PARSER_XFA_UTILS_H_
 
 #include "xfa/fde/xml/fde_xml.h"
@@ skipping 102 matching lines @@
         m_NodeStack.Push(pChildItem);
       }
       return *m_NodeStack.GetTopElement();
     }
     return NULL;
   }
   NodeType* MoveToNext() {
     NodeType** ppNode = NULL;
     NodeType* pCurrent = GetCurrent();
     while (m_NodeStack.GetSize() > 0) {
-      while ((ppNode = m_NodeStack.GetTopElement())) {
+      while ((ppNode = m_NodeStack.GetTopElement()) != nullptr) {
         if (pCurrent != *ppNode) {
           return *ppNode;
         }
         NodeType* pChild = TraverseStrategy::GetFirstChild(*ppNode);
         if (pChild == NULL) {
           break;
         }
         m_NodeStack.Push(pChild);
       }
-      while ((ppNode = m_NodeStack.GetTopElement())) {
+      while ((ppNode = m_NodeStack.GetTopElement()) != nullptr) {
         NodeType* pNext = TraverseStrategy::GetNextSibling(*ppNode);
         m_NodeStack.Pop();
         if (m_NodeStack.GetSize() == 0) {
           break;
         }
         if (pNext) {
           m_NodeStack.Push(pNext);
           break;
         }
       }
     }
     return NULL;
   }
   NodeType* SkipChildrenAndMoveToNext() {
-    NodeType** ppNode = NULL;
-    while ((ppNode = m_NodeStack.GetTopElement())) {
+    NodeType** ppNode = nullptr;
+    while ((ppNode = m_NodeStack.GetTopElement()) != nullptr) {
       NodeType* pNext = TraverseStrategy::GetNextSibling(*ppNode);
       m_NodeStack.Pop();
       if (m_NodeStack.GetSize() == 0) {
         break;
       }
       if (pNext) {
         m_NodeStack.Push(pNext);
         break;
       }
     }
@@ skipping 19 matching lines @@
 FX_BOOL XFA_IsLayoutElement(XFA_ELEMENT eElement,
                             FX_BOOL bLayoutContainer = FALSE);
 
 void XFA_DataExporter_DealWithDataGroupNode(CXFA_Node* pDataNode);
 void XFA_DataExporter_RegenerateFormFile(CXFA_Node* pNode,
                                          IFX_Stream* pStream,
                                          const FX_CHAR* pChecksum = NULL,
                                          FX_BOOL bSaveXML = FALSE);
 
 #endif  // XFA_FXFA_PARSER_XFA_UTILS_H_