Invalid write in threads::ThreadDelegate::~ThreadDelegate()

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Bug Report

A memory error occurs when a thread is destroyed before its delegate. The delegate will still hold a pointer to the thread and will try to write on that pointer at destruction.

Reproduction Steps

1. Start core with valgrind:
   LD_LIBRARY_PATH=. valgrind --trace-children=yes --leak-check=full ./smartDeviceLinkCore > valgrind.log 2>&1
2. Start SDL HMI
3. Wait patiently for the HMI to connect
4. Send Exit Application -> IGNITION_OFF

Expected Behavior

The valgrind log shows no memory errors

Observed Behavior

The valgrind log shows a similar memory error many times:

==92645== Invalid write of size 8
==92645==    at 0x719118: threads::Thread::set_delegate(threads::ThreadDelegate*) (thread.h:128)
==92645==    by 0x718C69: threads::ThreadDelegate::~ThreadDelegate() (thread_delegate.cc:44)

OS & Version Information

• OS/Version: Ubuntu 18.04.2
• SDL Core Version: develop
• Testing Against: sdl_hmi

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Closed via #3388