ASAN: thread_pool_server_audit reports heap-use-after-free.

Marko came upon this:
built as
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo -DWITH_ASAN=ON) and then run ASAN_OPTIONS=abort_on_error=1:disable_coredump=0 ./mtr --parallel=auto --repeat=100 plugins.thread_pool_server_audit

reports:

plugins.thread_pool_server_audit w2 [ pass ] 18147
worker[2] > Restart [mysqld.1 - pid: 181054, winpid: 181054] - running with different options '-thread_stack=655360' != '-thread_handling=pool-of-threads'
***Warnings generated in error logs during shutdown after running tests: plugins.thread_pool_server_audit
==181055==ERROR: AddressSanitizer: heap-use-after-free on address 0x608000000fbc at pc 0x56171b923903 bp 0x7ffe76c68d50 sp 0x7ffe76c68d40
SUMMARY: AddressSanitizer: heap-use-after-free /home/buildbot/buildbot/build/mariadb-10.4.16/sql/threadpool_common.cc:534 in tp_post_kill_notification