Uncontrollable memory consumption upon GROUP_CONCAT with ORDER BY

perl /mnt8t/src/rqg/run.pl --gendata=conf/zz/full_text_search.zz --nometadata --scenario=Standard --ps-protocol --filter=conf/ff/need-reconnect.ff  --queries=1000000 --gendata=advanced --reporters=Backtrace,ErrorLog,Deadlock,MemoryUsage,FeatureUsage --mysqld=--slave-transaction-retry-errors="1213,1205" --mysqld=--secure-timestamp=NO --mysqld=--table-open-cache-instances=64 --mysqld=--log-output=FILE,TABLE --threads=4 --duration=400 --mysqld=--max-statement-time=300 --mysqld=--lock-wait-timeout=300 --mysqld=--innodb-lock-wait-timeout=300 --views --grammar=conf/yy/optimizer.yy --gendata=simple --mysqld=--innodb-compression-algorithm=none --base-port=14000 --server1-basedir=/data/releases/10.6.9-5 --vardir=/home/elenst/logs/var3 --seed=1671514559