tls: fix performance issue

See https://github.com/orangemocha/node-connection-drop

I have pinpointed the performance degradation to
https://github.com/joyent/node/commit/ac2263b77f3f346458d06fc019de27e24c63cee0

This change brings performance back to the orginal levels.

Signed-off-by: Fedor Indutny <fedor@indutny.com>