Avoid the double-free (first in fts_read, second in fts_close) that

Avoid the double-free (first in fts_read, second in fts_close) that
would occur when an `active' directory is made inaccessible (e.g.,
via chmod a-x) during a traversal.

(fts_read): After a failed fchdir, update sp->fts_cur
before returning.  Reproduce this failure by
mkdir -p a/b; cd a; chmod a-x . b
Reported by Stavros Passas.