Finish unit test for CFX_ByteStringC class.

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 1064 matching lines @@
 }
 FX_DWORD CFX_ByteString::GetID(FX_STRSIZE start_pos) const
 {
     return CFX_ByteStringC(*this).GetID(start_pos);
 }
 FX_DWORD CFX_ByteStringC::GetID(FX_STRSIZE start_pos) const
 {
     if (m_Length == 0) {
         return 0;
     }
-    if (start_pos >= m_Length) {
+    if (start_pos < 0 || start_pos >= m_Length) {
         return 0;
     }
     FX_DWORD strid = 0;
     if (start_pos + 4 > m_Length) {
         for (FX_STRSIZE i = 0; i < m_Length - start_pos; i ++) {
             strid = strid * 256 + m_Ptr[start_pos + i];
         }
         strid = strid << ((4 - m_Length + start_pos) * 8);
     } else {
         for (int i = 0; i < 4; i ++) {
@@ skipping 73 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));
 }