amd64-windows-packages: MTR fails with "Could not parse mysqld --help: thread-" after 854869d69fd

Since commit 854869d69fd ("Fix a UBSAN runtime error on MacOSX", mysys/my_stack.c) the amd64-windows-packages builder fails on 10.11 before running any test:

perl mysql-test-run.pl --force --suite-timeout=120 --max-test-fail=10 --retry=3 --suite=vcol,gcol,perfschema,main,innodb,versioning,plugins,mariabackup,roles,auth_gssapi,rocksdb,sysschema

VS config: RelWithDebInfo

Checking supported features...

MariaDB Version 10.11.20-MariaDB

Could not parse mysqld --help: thread-

dojob: spawn returned error 255

The error comes from mysql-test/mariadb-test-run.pl (collect_mysqld_features()): a line of the "mysqld --help --verbose" variables list is just thread-, without a value. The output looks truncated in the middle of a variable name (thread-cache-size / thread-handling / thread-pool-* / thread-stack), which suggests mysqld dies while printing --help.

Builds (all builder amd64-windows-packages, id 239):

• 44116eca17c (MDEV-36295): build 48319 - OK
• 854869d69fd (Fix a UBSAN runtime error on MacOSX): build 48329 - fails (step "connect test")
• 6a063120d4d (MDEV-23864): build 48334 - fails (step "nm test")

https://buildbot.mariadb.org/#/builders/239/builds/48329
https://buildbot.mariadb.org/#/builders/239/builds/48334

854869d69fd is the only commit between the last green and the first red build. It changes the fallback path of my_get_stack_bounds():

-  *stack_end= (char *)(*stack_start) + stack_size * STACK_DIRECTION;

+  *stack_end= (char *)(*stack_start) + ((ssize_t)stack_size) * STACK_DIRECTION;

The plain amd64-windows builder (57066) is green on the same commit, so it is not reproducible on every Windows build.