Fix compilation warning due to a recent change

* src/json.c (json_handle_nonlocal_exit): Always return a
value.

src/json.c

@@ -674,6 +674,8 @@ json_handle_nonlocal_exit (enum nonlocal_exit type, Lisp_Object data)
       return data;
     case NONLOCAL_EXIT_THROW:
       return Fcons (Qno_catch, data);
+    default:
+      return Qnil;
     }
 }