Fix error handling in POSIX version of the base::File::GetLength.

sandbox/win/src/process_mitigations_test.cc

 // Copyright 2011 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 "sandbox/win/src/process_mitigations.h"
 
 #include <d3d9.h>
 #include <initguid.h>
 #include <opmapi.h>
 #include <psapi.h>
@@ skipping 1518 matching lines @@
 
   // Open font file passed in as an argument.
   base::File file(base::FilePath(argv[0]),
                   base::File::FLAG_OPEN | base::File::FLAG_READ);
   if (!file.IsValid())
     // Failed to open the font file passed in.
     return SBOX_TEST_NOT_FOUND;
 
   std::vector<char> font_data;
   int64_t len = file.GetLength();
+  if (len < 0)
+    return SBOX_TEST_NOT_FOUND;
   font_data.resize(len);
 
   int read = file.Read(0, &font_data[0], len);
   file.Close();
 
   if (read != len)
     return SBOX_TEST_NOT_FOUND;
 
   DWORD font_count = 0;
   HANDLE font_handle = add_font_mem_resource(
@@ skipping 254 matching lines @@
   cmd = cmd.Append(L"calc.exe");
 
   std::wstring test_command(base::StringPrintf(L"TestChildProcess %ls 0x%08X",
                                                cmd.value().c_str(),
                                                STATUS_ACCESS_VIOLATION));
 
   EXPECT_EQ(SBOX_TEST_SUCCEEDED, runner.RunTest(test_command.c_str()));
 }
 
 }  // namespace sandbox