Add check for trivial string equality

core/fxcrt/fx_basic_wstring.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 <stddef.h>
 
 #include <algorithm>
 #include <cctype>
@@ skipping 162 matching lines @@
 
 bool CFX_WideString::operator==(const CFX_WideStringC& str) const {
   if (!m_pData)
     return str.IsEmpty();
 
   return m_pData->m_nDataLength == str.GetLength() &&
          wmemcmp(m_pData->m_String, str.c_str(), str.GetLength()) == 0;
 }
 
 bool CFX_WideString::operator==(const CFX_WideString& other) const {
+  if (m_pData == other.m_pData)
+    return true;
+
   if (IsEmpty())
     return other.IsEmpty();
 
   if (other.IsEmpty())
     return false;
 
   return other.m_pData->m_nDataLength == m_pData->m_nDataLength &&
          wmemcmp(other.m_pData->m_String, m_pData->m_String,
                  m_pData->m_nDataLength) == 0;
 }
@@ skipping 807 matching lines @@
       FXSYS_MultiByteToWideChar(codepage, 0, bstr.c_str(), src_len, nullptr, 0);
   CFX_WideString wstr;
   if (dest_len) {
     FX_WCHAR* dest_buf = wstr.GetBuffer(dest_len);
     FXSYS_MultiByteToWideChar(codepage, 0, bstr.c_str(), src_len, dest_buf,
                               dest_len);
     wstr.ReleaseBuffer(dest_len);
   }
   return wstr;
 }