sort: speed up default full line sorting

sort: speed up default full line sorting

Don't write NUL after the comparison buffers on each compare,
which increases performance by about 3% for short lines
on a pentium-m with gcc-4.4.1

* src/sort.c: (fillbuf): Delimit input items with NUL.
(write_bytes): Restore the item delimiter char which was
replaced with NUL in fillbuf().