stream: Fix readableState.awaitDrain mechanism

In 68990948 (https://github.com/nodejs/node/pull/2325),
the conditions for increasing `readableState.awaitDrain` when
writing to a piping destination returns false were changed so
that they could not actually be met, effectively leaving
`readableState.awaitDrain` with a constant value of 0.

This patch changes the conditions to testing whether the
stream for which `.write()` returned false is still a piping
destination, which was likely the intention of the original
patch.

Fixes: https://github.com/nodejs/node/issues/5820
Fixes: https://github.com/nodejs/node/issues/5257
PR-URL: https://github.com/nodejs/node/pull/6023


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>