clang-8 leak in forder

[mattdowle]

Using clang-8 locally (8b00b5e), it shows a leak on exit :

> test.data.table()
# all ok
> q()
=================================================================
==26171==ERROR: LeakSanitizer: detected memory leaks
Direct leak of 14 byte(s) in 2 object(s) allocated from:
    #0 0x4dabca in calloc (/home/mdowle/build/R-devel-strict-clang/bin/exec/R+0x4dabca)
    #1 0x7ff8a6e20e99 in forder /tmp/RtmpDd48za/R.INSTALL651a28f8fb31/data.table/src/forder.c:600:25
    #2 0x66a960 in R_doDotCall /home/mdowle/build/R-devel-strict-clang/src/main/dotcode.c:586:17