Get rid of most instance of 'foo != NULL'

core/src/fxcrt/fx_basic_plex.cpp

 // 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
 
 #include "core/include/fxcrt/fx_memory.h"
 #include "plex.h"
 
 CFX_Plex* CFX_Plex::Create(CFX_Plex*& pHead,
                            FX_DWORD nMax,
                            FX_DWORD cbElement) {
   CFX_Plex* p =
       (CFX_Plex*)FX_Alloc(uint8_t, sizeof(CFX_Plex) + nMax * cbElement);
   p->pNext = pHead;
   pHead = p;
   return p;
 }
 void CFX_Plex::FreeDataChain() {
   CFX_Plex* p = this;
-  while (p != NULL) {
-    uint8_t* bytes = (uint8_t*)p;
-    CFX_Plex* pNext = p->pNext;
-    FX_Free(bytes);
-    p = pNext;
+  while (p) {
+    CFX_Plex* old = p;
+    p = p->pNext;
+    FX_Free(old);
   }
 }