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: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7344)

(cherry picked from commit d97ce8d9a0619c1d9d1222dc1b44dbebb58dd966)