Make %DebugPushPromise more robust wrt fuzzing.

src/debug.cc

Index: src/debug.cc
diff --git a/src/debug.cc b/src/debug.cc
index dec51b9954a5d15cfb04536cd317964335f9c0e8..2ae8630885b804640cd77f5e4363b62cfd5db8a6 100644
--- a/src/debug.cc
+++ b/src/debug.cc
@@ -1316,11 +1316,9 @@ Handle<Object> Debug::GetPromiseOnStackOnThrow() {
       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;
 }