uci: fix use-after-free uci_add_list

uci: fix use-after-free uci_add_list

When uci_add_list is called with ptr->o set and ptr->option = NULL,
then in uci_expand_ptr ptr->option is set to ptr->o->e.name.
If ptr->o->type is UCI_TYPE_STRING then prev is set to ptr->o.
This will result in use-after-free because ptr->option is used in
the call to uci_add_delta in uci_add_element_list after
uci_free_option(prev).

Signed-off-by: Jan Venekamp <[email protected]>