bug#26319: sort -u deletes characters it shouldn't

[sort-u]

sort (GNU coreutils) 8.26local of the system: en_US.UTF-8contents of 
/etc/locale.conf:LANG=en_US.UTF-8LC_TIME=fr_FR.UTF-8LC_NUMERIC=fr_FR.UTF-8========================================$
 echo $'䜌\n龹' | sort -uExpected output:龹䜌Actual output:䜌