Core/Threading: Fix race condition in Thread

Increment the reference count of m_task in Thread::start() before spawning the actual Thread that will execute the task, otherwise the thread might finish, decRef the task and delete it.

Valgrind log of the issue:
 Invalid read of size 8
   at 0x1314CAD: ACE_Atomic_Op_GCC<long>::operator++() (Atomic_Op_GCC_T.inl:34)
   by 0x15933FB: ACE_Based::Runnable::incReference() (Threading.h:36)
   by 0x1592D2D: ACE_Based::Thread::start() (Threading.cpp:136)
   by 0x1592C37: ACE_Based::Thread::Thread(ACE_Based::Runnable*) (Threading.cpp:111)
   by 0xF6C463: Master::Run() (Master.cpp:195)
   by 0xF725D0: main (Main.cpp:142)
 Address 0x26137278 is 8 bytes inside a block of size 24 free'd
   at 0x4C2B59C: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
   by 0xF67FDB: RARunnable::~RARunnable() (RARunnable.cpp:55)
   by 0x1593441: ACE_Based::Runnable::decReference() (Threading.h:40)
   by 0x1592E92: ACE_Based::Thread::ThreadTask(void*) (Threading.cpp:186)
   by 0x515EA35: ACE_OS_Thread_Adapter::invoke() (in /usr/lib/libACE-6.0.3.so)
   by 0x5F19F8D: start_thread (pthread_create.c:311)
   by 0x6A46E1C: clone (clone.S:113)

Closes #10619