We don't need to start the goroutines if the program isn't going to do
any I/O.

Fixes #19390.

Change-Id: I47eef992d3ad05ed5f3150f4d6e5b3e0cb16a551
Reviewed-on: https://go-review.googlesource.com/37762
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>