Fix edge case in stack trace formatting.

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 296965d37dd743891f4d7798582bbbb0dc341951..2ad68cbb032a8de3ffde75ab9938e40c2d600c55 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -906,8 +906,8 @@ function CallSiteGetPosition() {
 
 function CallSiteIsConstructor() {
   var receiver = this[CallSiteReceiverKey];
-  var constructor =
-      IS_OBJECT(receiver) ? %GetDataProperty(receiver, "constructor") : null;
+  var constructor = (receiver != null && IS_OBJECT(receiver))
+                        ? %GetDataProperty(receiver, "constructor") : null;
   if (!constructor) return false;
   return this[CallSiteFunctionKey] === constructor;
 }