Null-check prop before dropping it (156354, r=khanson, sr=shaver).

js/src/jsobj.c

@@ -1184,6 +1184,7 @@ obj_propertyIsEnumerable(JSContext *cx, JSObject *obj, uintN argc, jsval *argv,
     }
 
     ok = OBJ_GET_ATTRIBUTES(cx, obj, id, prop, &attrs);
+    if (prop)
         OBJ_DROP_PROPERTY(cx, obj2, prop);
     if (ok)
         *rval = BOOLEAN_TO_JSVAL((attrs & JSPROP_ENUMERATE) != 0);