Fix memory leak on (Gnu)TLS close.

Fix memory leak on (Gnu)TLS close.

This leak doesn't show up under normal operation, as the process
normally dies right after closing the session.

But during callout repetitive TLS sessions are opened and closed from
the same process (the process receiving the message). Depending on
the amount of RAM and the number of callouts the same process does,
this may be a problem. (On an amd64 machine with 4GB RAM, at about 1000
recipients the memory is exhausted.)