Use eassume (false) for branch that's never taken.

* src/json.c (json_handle_nonlocal_exit): Use eassume (false) since
this branch is never taken.

src/json.c

@@ -675,7 +675,7 @@ json_handle_nonlocal_exit (enum nonlocal_exit type, Lisp_Object data)
     case NONLOCAL_EXIT_THROW:
       return Fcons (Qno_catch, data);
     default:
-      return Qnil;
+      eassume (false);
     }
 }