fix modify after free crash

[ernstblechaPT]

mThreadHandle must not be reset before the Thread has rejoined

[azoitl]

This breaks restarting 4diac FORTE resources via management commands. There we only stop the thread and later want to start it again. If only stop is called then threadhandle is never set to nullHandle.

A potential solution is that end() cashes the threadhandle and resets it before join() and both end() and join() set it to nullHandle. Another option is that at the end of runThread() it is set to nullHandle

[ernstblechaPT]

Thank you for the feedback, i guess setting it to nullHandle at the end of runThread() is the most sensible solution

I wasn't sure if this might lead to a race if a thread is restarted later: until runThread() has finished the following start() will fail silently?