busybox

tar: handle the case when opened created tarball happens to have fd#0

Reproducer:
    exec 0>&-
    exec 1>&-
    tar czf z.tar.gz FILE

function                                             old     new   delta
vfork_compressor                                     229     257     +28

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>