This just reads everything remaining in the stream in a blocking way and will
be used to get the data remaining in the stream buffers after the child
process had been already closed (and hence can't write any more to it and
there is no risk of deadlock).
See #10258.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74343 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775