Fix the fix for the native symbol resolver on Windows.

runtime/vm/native_symbol_win.cc

 // Copyright (c) 2013, 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 "vm/globals.h"
 #if defined(HOST_OS_WINDOWS)
 
 #include "vm/lockers.h"
 #include "vm/native_symbol.h"
 #include "vm/os_thread.h"
@@ skipping 49 matching lines @@
   HANDLE hProcess = GetCurrentProcess();
   DWORD64 address = static_cast<DWORD64>(pc);
   PSYMBOL_INFO pSymbol = reinterpret_cast<PSYMBOL_INFO>(&buffer[0]);
   pSymbol->SizeOfStruct = kSymbolInfoSize;
   pSymbol->MaxNameLen = kMaxNameLength;
   DWORD64 displacement;
   BOOL r = SymFromAddr(hProcess, address, &displacement, pSymbol);
   if (r == FALSE) {
     return NULL;
   }
-  *start = pc - displacement;
+  if (start != NULL) {
+    *start = pc - displacement;
+  }
   return strdup(pSymbol->Name);
 }
 
 
 void NativeSymbolResolver::FreeSymbolName(char* name) {
   free(name);
 }
 
 
 bool NativeSymbolResolver::LookupSharedObject(uword pc,
                                               uword* dso_base,
                                               char** dso_name) {
   return false;
 }
 
 }  // namespace dart
 
 #endif  // defined(HOST_OS_WINDOWS)