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

core/fxcrt/fx_basic_gcc.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 <cctype>
 #include <cwctype>
 #include <limits>
 
@@ skipping 208 matching lines @@
                               uint32_t dwFlags,
                               const FX_WCHAR* wstr,
                               int wlen,
                               FX_CHAR* buf,
                               int buflen,
                               const FX_CHAR* default_str,
                               int* pUseDefault) {
   int len = 0;
   for (int i = 0; i < wlen; i++) {
     if (wstr[i] < 0x100) {
-      if (buf && len < buflen) {
-        buf[len] = (FX_CHAR)wstr[i];
-      }
+      if (buf && len < buflen)
+        buf[len] = static_cast<FX_CHAR>(wstr[i]);
       len++;
     }
   }
   return len;
 }
 int FXSYS_MultiByteToWideChar(uint32_t codepage,
                               uint32_t dwFlags,
                               const FX_CHAR* bstr,
                               int blen,
                               FX_WCHAR* buf,
                               int buflen) {
   int wlen = 0;
   for (int i = 0; i < blen; i++) {
     if (buf && wlen < buflen) {
       buf[wlen] = bstr[i];
     }
     wlen++;
   }
   return wlen;
 }
 #ifdef __cplusplus
 }
 #endif
 #endif