Bug 1015339 - Spot fix FormatStackDump. (r=bz)

js/src/vm/OldDebugAPI.cpp

@@ -1055,7 +1055,7 @@ FormatFrame(JSContext *cx, const NonBuiltinScriptFrameIter &iter, char *buf, int
 
     RootedValue thisVal(cx);
     AutoPropertyDescArray thisProps(cx);
-    if (iter.computeThis(cx)) {
+    if (iter.hasUsableAbstractFramePtr() && iter.computeThis(cx)) {
         thisVal = iter.computedThisValue();
         if (showThisProps && thisVal.isObject()) {
             RootedObject thisObj(cx, &thisVal.toObject());