Test fclose on an unopened file.

Add new file libio/tst-fclosed-unopened.c that tests whether fclose on
an unopened file returns EOF.

Calling fclose on unopened files normally causes a use-after-free bug,
however the standard streams are an exception since they are not
deallocated by fclose.

fclose returning EOF for unopened files is not part of the external
contract but there are dependancies on this behaviour.  For example,
gnulib's close_stdout in lib/closeout.c.

Tested for x86_64.

Signed-off-by: Aaron Merey <amerey@redhat.com>