Getting rid of more (FX_LPCWSTR) casts and fixing two bugs revealed by this.

core/src/fxcrt/fx_basic_bstring.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 "../../include/fxcrt/fx_basic.h"
 #include "../../../third_party/numerics/safe_math.h"
 
 static int _Buffer_itoa(char* buf, int i, FX_DWORD flags)
@@ skipping 953 matching lines @@
 {
     if (len < 0) {
         len = (FX_STRSIZE)FXSYS_wcslen(str);
     }
     CFX_ByteString bstr;
     bstr.ConvertFrom(CFX_WideString(str, len));
     return bstr;
 }
 CFX_ByteString CFX_ByteString::FromUnicode(const CFX_WideString& str)
 {
-    return FromUnicode((FX_LPCWSTR)str, str.GetLength());
+    return FromUnicode(str.c_str(), str.GetLength());
 }
 void CFX_ByteString::ConvertFrom(const CFX_WideString& str, CFX_CharMap* pCharMap)
 {
     if (pCharMap == NULL) {
         pCharMap = CFX_CharMap::GetDefaultMapper();
     }
     *this = (*pCharMap->m_GetByteString)(pCharMap, str);
 }
 int CFX_ByteString::Compare(FX_BSTR str) const
 {
@@ skipping 184 matching lines @@
     FX_CHAR* pBuffer = (FX_CHAR*)(this + 1);
     FXSYS_memcpy32(pBuffer + m_Size, str.GetPtr(), len);
     m_Size += len;
 }
 void CFX_StringBufBase::Append(int i, FX_DWORD flags)
 {
     char buf[32];
     int len = _Buffer_itoa(buf, i, flags);
     Append(CFX_ByteStringC(buf, len));
 }