wc: fix regression determining file size

wc: fix regression determining file size

* src/wc.c (wc): Use off_t rather than size_t
when calculating where to seek to, so that
we don't seek to a too low offset on systems
where size_t < off_t, which would result in
many read() calls to determine the file size.
* tests/misc/wc-proc.sh: Add a test case
sufficient for 32 bit systems at least.
* NEWS: Mention the bug fix.
Reported at https://bugs.debian.org/1027101