util/cutils: Clean up control flow around qemu_strtol() a bit

Reorder check_strtox_error() to make it obvious that we always store
through a non-null @endptr.

Transform

    if (some error) {
        error case ...
        err = value for error case;
    } else {
        normal case ...
        err = value for normal case;
    }
    return err;

to

    if (some error) {
        error case ...
        return value for error case;
    }
    normal case ...
    return value for normal case;

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-9-git-send-email-armbru@redhat.com>
