Guard against the call to write() inside pipe's ondata pushing more data
back onto the Readable, thus causing ondata to be called again.
This is fine but results in awaitDrain being increased more than once.
The problem with that is when the destination does drain, only a single
'drain' event is emitted, so awaitDrain in this case will never reach
zero and we end up with a permanently paused stream.
Fixes: https://github.com/nodejs/node/issues/7278
PR-URL: https://github.com/nodejs/node/pull/7292
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
