Fix PKCS7_verify to not have an error stack if it succeeds.

Fix PKCS7_verify to not have an error stack if it succeeds.

Revert a change in behavior to BIO_write(). If a NULL BIO
is passed, no error is raised and the return value is 0. There are
many places where the return code from the write was not checked,
resulting in an error stack with no error status being returned.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15493)