Add another test for fclose on an unopened file

Add another test for fclose on an unopened file

Add new file libio/tst-fclose-unopened2.c that tests whether fclose on an
unopened file returns EOF.

This test differs from tst-fclose-unopened.c by ensuring the file's buffer
is allocated prior to double-fclose.  A comment in tst-fclose-unopened.c
now clarifies that it is testing a file with an unallocated buffer.

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.

Tested for x86_64.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>