Replace pthread_exit(NULL) calls by return NULL

This MR solves a segfault problem with pthread. See the linked issue for details. It follows the accepted answer in https://stackoverflow.com/a/27235846

Closes #121 (closed)

This seems to solve the issues when tested with clang libc++ (where the error actually showed up)

requested review from @markus.blatt

assigned to @simon.praetorius

changed the description

Related system test pipeline: https://gitlab.dune-project.org/infrastructure/dune-nightly-test/-/pipelines/74098 (passed)

approved this merge request

merged

mentioned in commit 3d34161d