test: fix infinite spawn cycle in stdio test

Fix parallel/test-stdio-closed introduced in commit b5f25a96 ("src: ensure that file descriptors 0-2 are valid") to not keep spawning child processes ad infinitum.

The test spawns itself as a child process but a missing return statement made the child process spawn itself again, and again, and again.

It went unnoticed for some time because the child process exits almost immediately afterwards, i.e. it didn't fill up the process table. The observable effect was an iojs process that was quietly consuming CPU cyles in the background with a PID that was constantly changing.

Refs: https://github.com/iojs/io.js/pull/938

R=@indutny

https://jenkins-iojs.nodesource.com/view/iojs/job/iojs+any-pr+multi/194/