Beef up unit test for CFX_ByteString and CFX_WideString.

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 423 matching lines @@
     nCount = 0;
   }
   if (nCount >= m_pData->m_nDataLength) {
     return *this;
   }
   CFX_WideString dest;
   AllocCopy(dest, nCount, 0);
   return dest;
 }
 CFX_WideString CFX_WideString::Mid(FX_STRSIZE nFirst) const {
+  if (!m_pData)
+    return CFX_WideString();
+
   return Mid(nFirst, m_pData->m_nDataLength - nFirst);
 }
 CFX_WideString CFX_WideString::Mid(FX_STRSIZE nFirst, FX_STRSIZE nCount) const {
-  if (!m_pData) {
+  if (!m_pData)
     return CFX_WideString();
-  }
-  if (nFirst < 0) {
-    nFirst = 0;
-  }
-  if (nCount < 0) {
-    nCount = 0;
-  }
-  if (nFirst + nCount > m_pData->m_nDataLength) {
-    nCount = m_pData->m_nDataLength - nFirst;
-  }
-  if (nFirst > m_pData->m_nDataLength) {
-    nCount = 0;
-  }
-  if (nFirst == 0 && nFirst + nCount == m_pData->m_nDataLength) {
+
+  nFirst = std::min(std::max(nFirst, 0), m_pData->m_nDataLength);
+  nCount = std::min(std::max(nCount, 0), m_pData->m_nDataLength - nFirst);
+  if (nFirst == 0 && nCount == m_pData->m_nDataLength)
     return *this;
-  }
+
   CFX_WideString dest;
   AllocCopy(dest, nCount, nFirst);
   return dest;
 }
 CFX_WideString CFX_WideString::Right(FX_STRSIZE nCount) const {
   if (!m_pData) {
     return CFX_WideString();
   }
   if (nCount < 0) {
     nCount = 0;
@@ skipping 566 matching lines @@
   int dest_len =
       FXSYS_MultiByteToWideChar(codepage, 0, bstr, src_len, nullptr, 0);
   CFX_WideString wstr;
   if (dest_len) {
     FX_WCHAR* dest_buf = wstr.GetBuffer(dest_len);
     FXSYS_MultiByteToWideChar(codepage, 0, bstr, src_len, dest_buf, dest_len);
     wstr.ReleaseBuffer(dest_len);
   }
   return wstr;
 }