Fix null pointer deref in message_handler.cc

runtime/vm/message_handler.cc

 // Copyright (c) 2011, 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/message_handler.h"
 
 #include "vm/dart.h"
 #include "vm/lockers.h"
 #include "vm/port.h"
 
@@ skipping 129 matching lines @@
 bool MessageHandler::HandleMessages(bool allow_normal_messages,
                                     bool allow_multiple_normal_messages) {
   // If isolate() returns NULL StartIsolateScope does nothing.
   StartIsolateScope start_isolate(isolate());
 
   // TODO(turnidge): Add assert that monitor_ is held here.
   bool result = true;
   Message::Priority min_priority = (allow_normal_messages && !paused()) ?
       Message::kNormalPriority : Message::kOOBPriority;
   Message* message = DequeueMessage(min_priority);
-  intptr_t message_len = message->len();
   while (message != NULL) {
+    intptr_t message_len = message->len();
     if (FLAG_trace_isolates) {
       OS::Print("[<] Handling message:\n"
                 "\tlen:        %" Pd "\n"
                 "\thandler:    %s\n"
                 "\tport:       %" Pd64 "\n",
                 message_len, name(), message->dest_port());
     }
 
     // Release the monitor_ temporarily while we handle the message.
     // The monitor was acquired in MessageHandler::TaskCallback().
@@ skipping 163 matching lines @@
 
 void MessageHandler::decrement_live_ports() {
   MonitorLocker ml(&monitor_);
 #if defined(DEBUG)
   CheckAccess();
 #endif
   live_ports_--;
 }
 
 }  // namespace dart