Make it safe to call close_file on an already closed file.

Make it safe to call close_file on an already closed file.

The SYSLINUX internal API closes a file automatically upon reading
to block EOF (not necessarily file EOF!)  Thus, it's fairly likely
we'll get SI == 0 (closed file) passed to close_file at some point
or another.

Just plain admit it, and make close_file accept SI == 0 by spec.