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 95b780c1d0549678c0a244c6e2112ec97edf0839 ("stdio: Add more setvbuf tests").