Check fflush on BIO_ctrl call

Bug found and fix suggested by Julian Rüth.
Push error if fflush fails

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3266

)
(cherry picked from commit 595b2a42)
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3348)