Don't crash on SIGPROF from foreign sources.

runtime/vm/thread_interrupter.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/thread_interrupter.h"
 
 #include "vm/flags.h"
 #include "vm/lockers.h"
 #include "vm/os.h"
 #include "vm/simulator.h"
@@ skipping 208 matching lines @@
   }
 }
 
 
 InterruptableThreadState* ThreadInterrupter::GetCurrentThreadState() {
   return _EnsureThreadStateCreated();
 }
 
 
 InterruptableThreadState* ThreadInterrupter::CurrentThreadState() {
-  return Thread::Current()->thread_state();
+  Thread* thread = Thread::Current();
+  return (thread == NULL) ? NULL : thread->thread_state();
 }
 
 
 void ThreadInterrupter::SetCurrentThreadState(InterruptableThreadState* state) {
   Thread::Current()->set_thread_state(state);
 }
 
 
 void ThreadInterruptNoOp(const InterruptedThreadState& state, void* data) {
   // NoOp.
@@ skipping 72 matching lines @@
   }
   {
     // Signal to main thread we are exiting.
     MonitorLocker shutdown_ml(monitor_);
     thread_running_ = false;
     shutdown_ml.Notify();
   }
 }
 
 }  // namespace dart