qemu-option: Drop dead assertion

Commit c6ecec43 "qemu-option: Check return value instead of @err
where convenient" simplified

    opts = qemu_opts_create(list, qdict_get_try_str(qdict, "id"), 1,
                            &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return NULL;
    }

to

    opts = qemu_opts_create(list, qdict_get_try_str(qdict, "id"), 1, errp);
    if (!opts) {
        return NULL;
    }

but neglected to delete

    assert(opts != NULL);

Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210610085026.436081-1-armbru@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>