Fix error handling when current working directory is deleted.

runtime/bin/directory_win.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/directory.h"
 #include "bin/file.h"
 #include "bin/utils.h"
@@ skipping 333 matching lines @@
 Directory::ExistsResult Directory::Exists(const char* dir_name) {
   const wchar_t* system_name = StringUtils::Utf8ToWide(dir_name);
   Directory::ExistsResult result = ExistsHelper(system_name);
   free(const_cast<wchar_t*>(system_name));
   return result;
 }
 
 
 char* Directory::Current() {
   int length = GetCurrentDirectoryW(0, NULL);
+  if (length == 0) return NULL;
   wchar_t* current = new wchar_t[length + 1];
   GetCurrentDirectoryW(length + 1, current);
   char* result = StringUtils::WideToUtf8(current);
   delete[] current;
   return result;
 }
 
 
 bool Directory::SetCurrent(const char* path) {
   const wchar_t* system_path = StringUtils::Utf8ToWide(path);
@@ skipping 104 matching lines @@
       MoveFileExW(system_path, system_new_path, flags);
   free(const_cast<wchar_t*>(system_path));
   free(const_cast<wchar_t*>(system_new_path));
   return (move_status != 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)