Fix ossl_param_bld_push_{utf8,octet}_string() / param_bld_convert()

Fix ossl_param_bld_push_{utf8,octet}_string() / param_bld_convert()

ossl_param_bld_push_{utf8,octet}_string() saved the constant string
pointer to a non-constant structure field, so we change that field to
a pointer to a constant.  We also modify param_bld_convert() to
pretend the resulting pointer for PTR types points to a constant as
well.

Completes #9649

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9651)