Bug 1245671: Fix Assert.rejects on release builds with DOM promises. r=markh

--HG--
extra : commitid : ApnwqfMotvd
extra : rebase_source : 371cc2eed07fc7ae801a1efb50b0187bf367776a

testing/modules/Assert.jsm

@@ -109,7 +109,7 @@ Assert.AssertionError = function(options) {
   let stack = Components.stack;
   do {
     stack = stack.asyncCaller || stack.caller;
-  } while(stack.filename && stack.filename.includes("Assert.jsm"))
+  } while(stack && stack.filename && stack.filename.includes("Assert.jsm"))
   this.stack = stack;
 };