bfdtest1 loop check

Add a check that next_archived_file doesn't return the same element.
Seen with the following, which I think shows a bug with "ar r" and
thin archives as you get two copies of artest.a in artest2.a.

$ rm tmpdir/artest*
$ ./ar rc tmpdir/artest.a tmpdir/bintest.o
$ ./ar rcT tmpdir/artest2.a tmpdir/artest.a
$ ./bfdtest1 tmpdir/artest.a
$ ./bfdtest1 tmpdir/artest2.a
$ ./ar rcT tmpdir/artest2.a tmpdir/artest.a
$ ./bfdtest1 tmpdir/artest2.a
oops: next_archived_file