Fix the BIO callback return code handling

The BIO callback handling incorrectly wrote over the return code passed
to the callback, meaning that an incorrect result was (eventually) returned
to the caller.

Fixes #7343

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7344)

(cherry picked from commit d97ce8d9)