Fix the code that causes warnings

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 876 matching lines @@
   if (!m_pData || pTargets.IsEmpty())
     return;
 
   FX_STRSIZE len = GetLength();
   if (len < 1)
     return;
 
   FX_STRSIZE pos = 0;
   while (pos < len) {
     FX_STRSIZE i = 0;
-    while (i < pTargets.GetLength() && pTargets[i] != m_pData->m_String[pos]) {
+    while (i < pTargets.GetLength() &&
+           pTargets.CharAt(i) != m_pData->m_String[pos]) {
       i++;
     }
     if (i == pTargets.GetLength()) {
       break;
     }
     pos++;
   }
   if (pos) {
     ReallocBeforeWrite(len);
     FX_STRSIZE nDataLength = len - pos;
@@ skipping 79 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;
 }