_hx509_unparse_utf8_string_name() and
_hx509_unparse_KRB5PrincipalName() can return a strpool even if
they fail. The strpool must be passed through rk_strpoolcollect()
in order to return the contents to the caller of
hx509_request_get_san().
Change-Id: Ifda5620f4e9e00ca188aa138f692fccc12621ae8