tls: fix throughput issues after incorrect merge

1e066e4a was done incorrectly and has overwritten an important change
in: c17449df. Using bigger output buffer increases performance in 3-4
times.

PR-URL: https://github.com/nodejs/node/pull/2381
Fix: https://github.com/joyent/node/issues/25803


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>