When too much data is available on stdin, it might not be
forwarded immediatly to the task (write() might fail with -EAGAIN),
so when stdin is terminated, there might be some remaining data
to be pushed to the task. In this case, delay the release of the sink
so no data is discarded.
Refs open-mpi/ompi#4744
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>