sort: simpler fix for sort -u data-loss bug, and for a FMR bug

sort: simpler fix for sort -u data-loss bug, and for a FMR bug

This also fixes a free-memory-read (FMR) bug: when fillbuf's realloc
of buf->buf frees the buffer into which saved_line.text points,
the processing of that just-read longer line includes comparison
against the saved line in freed memory.
* src/sort.c (overlap): Remove.
(fillbuf): Do not try to copy saved lines, as that is too risky
in the presence of parallelism, reallocated buffers, etc.
(sort): Invalidate any saved line before sorting a new batch.