Fix a BIO leak when verifying SSL certificates or calling get_peer_cert.

You can verify this with a modification of the trivial `start_tls` example program. Prior to this patch, the following program's memory usage will grow unboundedly.

```
require 'eventmachine'

module Handler
  def post_init
    puts "Starting TLS"
    start_tls
  end

  def ssl_handshake_completed
    puts "Connected. Cert:"
    puts get_peer_cert
    loop { get_peer_cert }
  end

  def unbind
    EventMachine::stop_event_loop
  end
end

EventMachine.run do
  EventMachine.connect "mail.google.com", 443, Handler
end
```