Fix SSL_MODE_RELEASE_BUFFERS functionality

At some point in the past do_ssl3_write() used to return the number of
bytes written, or a value <= 0 on error. It now just returns a success/
error code and writes the number of bytes written to |tmpwrit|.

The SSL_MODE_RELEASE_BUFFERS code was still looking at the return code
for the number of bytes written rather than |tmpwrit|. This has the effect
that the buffers are not released when they are supposed to be.

Fixes #9490

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9505)

(cherry picked from commit 8bbf63e48f27c5edaa03e6d87d969c9b6a207f3c)