s4:kdc: Have smb_krb5_principal_get_comp_string() properly indicate an error

s4:kdc: Have smb_krb5_principal_get_comp_string() properly indicate an error

The existing implementation did not differentiate between the case where
the relevant component was not present, and that where talloc_strndup()
failed. To correct this situation, put the result into an out parameter
on success and return an error on failure.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15482

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>