Segmentation fault when using pthread_exit(NULL)

In the system test with clang and libc++ the dune-istl jobs are failing in all pythreadamgxyz tests with a segmentation fault, see

• https://gitlab.dune-project.org/infrastructure/dune-nightly-test/-/jobs/614819

It seems to be related to the call to pthread_exit(NULL) and not returning anything from a function. This issue might be related to

• https://stackoverflow.com/questions/27235502/segmentation-fault-on-terminating-a-thread-using-pthread-exit

Similar errors are reported

• https://github.com/namhyung/uftrace/issues/242
• https://github.com/namhyung/uftrace/issues/826

I guess it was just not showing an error so far, because relying on undefined behavior?