MDEV-22035 Memory leak in main.mysqltest

The test main.mysqltest could crash or hang with
cmake -DWITH_ASAN=ON builds. The reason appears to be
a memory leak, which was found out by manually invoking

echo --replace_regex a > file
ASAN_OPTIONS=log_path=/dev/shm/asan mysqltest ... < file

and then examining the /dev/shm/asan.* file.