The helper thread may get canceled before the open system
call succeds. Then ThreadData.fd remains zero, and eventually
the xclose call in end_reader_thread fails because descriptor 0
is not open.
Instead, initialize the fd member to -1 (not a valid descriptor)
and close the descriptor only if valid. Do this in a new end_thread
helper routine.
Also add more error checking to close operations.
Fixes commit 95b780c1d0 ("stdio: Add
more setvbuf tests").
Florian Weimer
DJ Delorie