stdio-common: In tst-setvbuf2, close helper thread descriptor only if opened

stdio-common: In tst-setvbuf2, close helper thread descriptor only if opened

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").