Avoid a write to NULL in ReadFieldBinary

chrome/browser/policy/preg_parser_win.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/policy/preg_parser_win.h"
 
 #include <windows.h>
 
 #include <algorithm>
 #include <iterator>
@@ skipping 48 matching lines @@
   int result = **cursor | (*(*cursor + 1) << 8);
   *cursor += sizeof(char16);
   return result;
 }
 
 // Reads a fixed-size field from a PReg file.
 bool ReadFieldBinary(const uint8** cursor,
                      const uint8* end,
                      int size,
                      uint8* data) {
+  if (!size)
+    return false;
   const uint8* field_end = *cursor + size;
   if (field_end > end)
     return false;
   std::copy(*cursor, field_end, data);
   *cursor = field_end;
   return true;
 }
 
 bool ReadField32(const uint8** cursor, const uint8* end, uint32* data) {
   uint32 value = 0;
@@ skipping 215 matching lines @@
   }
 
   LOG(ERROR) << "Error parsing " << file_path.value() << " at offset "
              << reinterpret_cast<const uint8*>(cursor - 1) - mapped_file.data();
   status->Add(POLICY_LOAD_STATUS_PARSE_ERROR);
   return false;
 }
 
 }  // namespace preg_parser
 }  // namespace policy