Make %DebugPushPromise more robust wrt fuzzing.

src/debug.cc

 // Copyright 2012 the V8 project 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 "src/v8.h"
 
 #include "src/api.h"
 #include "src/arguments.h"
 #include "src/bootstrapper.h"
 #include "src/code-stubs.h"
@@ skipping 1298 matching lines @@
   // Find the top-most try-catch handler.
   StackHandler* handler = StackHandler::FromAddress(
       Isolate::handler(isolate_->thread_local_top()));
   do {
     if (handler == promise_try) {
       // Mark the pushed try-catch handler to prevent a later duplicate event
       // triggered with the following reject.
       return thread_local_.promise_on_stack_->promise();
     }
     handler = handler->next();
-    // There must be a try-catch handler if a promise is on stack.
-    DCHECK_NE(NULL, handler);
     // Throwing inside a Promise can be intercepted by an inner try-catch, so
     // we stop at the first try-catch handler.
-  } while (!handler->is_catch());
+  } while (handler != NULL && !handler->is_catch());
   return undefined;
 }
 
 
 bool Debug::PromiseHasRejectHandler(Handle<JSObject> promise) {
   Handle<JSFunction> fun = Handle<JSFunction>::cast(
       JSObject::GetDataProperty(isolate_->js_builtins_object(),
                                 isolate_->factory()->NewStringFromStaticAscii(
                                     "PromiseHasRejectHandler")));
   Handle<Object> result =
@@ skipping 2084 matching lines @@
   logger_->DebugEvent("Put", message.text());
 }
 
 
 void LockingCommandMessageQueue::Clear() {
   base::LockGuard<base::Mutex> lock_guard(&mutex_);
   queue_.Clear();
 }
 
 } }  // namespace v8::internal