fix add js_pushundefined() in dbg_js_lsf()

debugus.c

@@ -412,6 +412,8 @@ void dbg_js_lsf(js_State *js)
             libelfin_wrap_free_info(ai);
         }
     }
+
+    js_pushundefined(js);
 }
 
 void dbg_js_lif(js_State *js)