Fix thread joining in dt_control_shutdown()

[jenshannoschwalm]

The last thread of num_threads clearly missed joining. Introduced in 77bed88

@dterrahe hinted to this in #20002, i can't remember how that happened in 77bed88 and if/how i oversaw the mentioned discussion.

[jenshannoschwalm]

Also safe for 5.4.1

[jenshannoschwalm]

@TurboGit i checked this again, it's pretty hard to trigger an "issue" as you have to add a job running in num_threads-1 but in that case the job thread is not joined so it's "running-in-the-wild" after control closing leading to an invisible crash while dt is closing. So for me also for 5.4.1

[TurboGit]

Thanks!