(fts_read): When about to fail (by returning NULL) due

(fts_read): When about to fail (by returning NULL) due
to a failed fchdir or failed fts_safe_changedir call, set
`sp->fts_cur = p'.  Do this by removing the explicit `return NULL;'
statements and setting p->fts_errno so execution falls through
to the common-case code below.  Otherwise, after such a failure,
calling fts_close would attempt to free an already-freed buffer.
Reported by Luis Lopez Lopez in http://bugs.debian.org/276352.