Add CFX_ByteStringC::CharAt() to avoid c_str() and casts

core/fpdfapi/fpdf_font/cpdf_font.cpp

 // Copyright 2016 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/fpdfapi/fpdf_font/include/cpdf_font.h"
 
 #include <memory>
 
@@ skipping 131 matching lines @@
   const CPDF_CIDFont* pCIDFont = AsCIDFont();
   if (pCIDFont) {
     bVertWriting = pCIDFont->IsVertWriting();
   } else {
     bVertWriting = m_Font.IsVertical();
   }
   return bVertWriting;
 }
 
 int CPDF_Font::AppendChar(FX_CHAR* buf, uint32_t charcode) const {
-  *buf = (FX_CHAR)charcode;
+  *buf = static_cast<FX_CHAR>(charcode);
   return 1;
 }
 
 void CPDF_Font::AppendChar(CFX_ByteString& str, uint32_t charcode) const {
   char buf[4];
   int len = AppendChar(buf, charcode);
   if (len == 1) {
     str += buf[0];
   } else {
     str += CFX_ByteString(buf, len);
@@ skipping 299 matching lines @@
   if (charcode < 0 || charcode >= 256)
     return nullptr;
 
   const FX_CHAR* name = nullptr;
   if (pCharNames)
     name = pCharNames[charcode].c_str();
   if ((!name || name[0] == 0) && iBaseEncoding)
     name = PDF_CharNameFromPredefinedCharSet(iBaseEncoding, charcode);
   return name && name[0] ? name : nullptr;
 }