libdwfl/offline.c: Avoid closing invalid fd

libdwfl/offline.c: Avoid closing invalid fd

process_archive may be called with an fd argument of -1, which
libelf interprets as "no file opened".  However when closing
the fd process_archive does not check whether the fd is valid
and may attempt to close an fd of -1.

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