busybox

tar: fix handling of first argument without '-'

The following no longer works as expected:

   $ ./busybox tar xfz test.tgz
   tar: can't open 'z': No such file or directory

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>