zlib: only apply drain listener if given callback

When stream.flush() is called without a callback, an empty listener is
being added. Since flush may be called multiple times to push SSE's
down to the client, multiple noop listeners are being added. This in
turn causes the memory leak detected message.

PR-URL: https://github.com/nodejs/node/pull/3534


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>