ui/ncurses: Don't free item in pmenu_item_setup

Currently pmenu_item_setup may free its item parameter on error.

This makes it non-obvious whether the item is still allocated on exit to
the caller.

Instead, this change removes the talloc_free, and requires that the
caller do this on error. This makes the potential use-after-free in
cui_boot_editor_on_exit obvious, so we fix that too.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>