Fix memory leaks after failure of PKCS7_add_signed_attribute()

Fix memory leaks after failure of PKCS7_add_signed_attribute()

If PKCS7_add_signed_attribute fails,
seq never escapes out of the callee and will
therefore result in a memory leak.
This is similar to ed3d277127.

CLA: trivial

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27670)