Fix ordertab() segfault when nunique is incorrectly supplied - #207

* Fix: ordertab() segfaults when nunique is smaller than actual

The ordertab() C function was susceptible to a buffer overflow when the
'nunique' parameter passed from R was smaller than the actual number of
unique values in the data. This led to intermittent segfaults due to
out-of-bounds writes to the 'ret' array.

This commit introduces a bounds check in the C implementation of
ordertab() (r_ram_integer64_ordertab_asc). If the number of unique
values encountered exceeds the allocated size of the 'ret' vector,
the loop is now terminated early, preventing any out-of-bounds writes
and thus eliminating the segfault. The function will now return a
truncated result in such cases, consistent with R's usual handling
of mismatched lengths.

A new test case has been added to test-sortuse64.R to specifically
target this scenario, using gctorture() to reliably trigger the
condition. This test now passes, confirming the fix.

Updated NEWS.md to reflect this bug fix.

* To taste

* tighten test

* comment purpose

* citation