[GH #399] Fix void istream_device::read function returning a value

From: Mateusz Loskot (mateusz_at_[hidden])
Date: 2019-10-28 22:30:01

Stefan,

In case you are going to merge `develop` to `master` very soon now,
please, wait until CI-s for this PR are done and the PR is merged.

https://github.com/boostorg/gil/pull/399

p.s. I know we still suffer from flood of warnings, it is annoying to read
through and we all ignore it by default (due to the flood),
but still skimming the warnings can find bugs.

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net