stream: fix y.pipe(x)+y.pipe(x)+y.unpipe(x)

Fix the uncommon situation when a readable stream is piped twice into
the same destination stream, and then unpiped once.

Previously, the `unpipe` event handlers weren’t able to tell whether
they were corresponding to the “right” conceptual pipe that was being
removed; this fixes this by adding a counter to the `unpipe` event
handler and only removing a single piping destination at most.

Fixes: https://github.com/nodejs/node/issues/12718
PR-URL: https://github.com/nodejs/node/pull/12746


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>