sort: avoid unaligned access.

sort: avoid unaligned access.
* src/sort.c (fillbuf): When enlarging the line buffer, ensure that
the new size is a multiple of "sizeof (struct line)".  This avoids
alignment problems when indexing from the end of the buffer.
Problem reported by Andreas Schwab in
<http://lists.gnu.org/archive/html/bug-coreutils/2007-07/msg00158.html>.