wc(1): fix NULL pointer dereference in cnt()

wc(1): fix NULL pointer dereference in cnt()

If the "file" argument to cnt() is NULL and we call warn(3) we will
get a NULL dereference.

Change the name of the argument to "path" and make "file" a local
variable.  Ensure that we set "file" to a valid C-string, even if
"path" is NULL.

While we're here, const the file name pointers, too.

Thread: https://marc.info/?l=openbsd-tech&m=163708784422157&w=2

ok millert@