box: fix a crash on unknown function option

`func_opts_reg` definition misses a `OPT_END` termintator item. This
leads to UB on iterating it. Particularly when `func_opts_reg` is used
as argument to `opts_decode` in `func_def_new_from_tuple`.

Closes #8463

NO_DOC=bugfix

(cherry picked from commit a652b03f)