Fix crash from turning on DevTools in the middle of catch prediction

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 01d9b0df1d7ae8ba6a5425955b0092cc3723ad9a..d25cf6112a80cc2ad9ff5a8cfb45956a140ac6e4 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -1751,18 +1751,20 @@ Handle<Object> Isolate::GetPromiseOnStackOnThrow() {
         }
         return retval;
       case HandlerTable::PROMISE:
-        return promise_on_stack->promise();
+        return promise_on_stack
+                   ? Handle<Object>::cast(promise_on_stack->promise())
+                   : undefined;
       case HandlerTable::ASYNC_AWAIT: {
         // If in the initial portion of async/await, continue the loop to pop up
         // successive async/await stack frames until an asynchronous one with
         // dependents is found, or a non-async stack frame is encountered, in
         // order to handle the synchronous async/await catch prediction case:
         // assume that async function calls are awaited.
-        retval = promise_on_stack->promise();
+        if (promise_on_stack) retval = promise_on_stack->promise();

[gsathya, 2016/09/24 01:14:09]

Can you not return early if you hit an undefined? 

[Dan Ehrenberg, 2016/09/24 03:08:20]

You're right, that's a cleaner way to do it. Fixed.

         if (PromiseHasUserDefinedRejectHandler(retval)) {
           return retval;
         }
-        promise_on_stack = promise_on_stack->prev();
+        if (promise_on_stack) promise_on_stack = promise_on_stack->prev();
         continue;

[gsathya, 2016/09/24 01:14:09]

Do you need this continue?

[Dan Ehrenberg, 2016/09/24 03:08:20]

In this switch statement, each case ends with either return or continue. I think
the pattern makes the code easier to read because it means that the reader
doesn't have to look to the end of the loop to figure out what's going to
happen.

       }
     }