Fix error when joining exited threads

The internal thread handles were cleared prematurely when attempting to join them. The handles should be also cleared when trying to kill already exited threads.

Source/Engine/Content/Loading/ContentLoadingManager.cpp

@@ -55,8 +55,7 @@ LoadingThread::~LoadingThread()
     // Check if has thread attached
     if (_thread != nullptr)
     {
-        if (_thread->IsRunning())
-            _thread->Kill(true);
+        _thread->Kill(true);
         Delete(_thread);
     }
 }

Source/Engine/Platform/Base/ThreadBase.cpp

@@ -43,7 +43,10 @@ void ThreadBase::SetPriority(ThreadPriority priority)
 void ThreadBase::Kill(bool waitForJoin)
 {
     if (!_isRunning)
+    {
+        ClearHandleInternal();
         return;
+    }
     ASSERT(GetID());
     const auto thread = static_cast<Thread*>(this);
 
@@ -105,7 +108,6 @@ int32 ThreadBase::Run()
         _callAfterWork = false;
         _runnable->AfterWork(false);
     }
-    ClearHandleInternal();
     _isRunning = false;
     ThreadExiting(thread, exitCode);
     ThreadRegistry::Remove(thread);

Source/Engine/Platform/Unix/UnixThread.cpp

@@ -53,6 +53,7 @@ UnixThread* UnixThread::Setup(UnixThread* thread, uint32 stackSize)
 void UnixThread::Join()
 {
     pthread_join(_thread, nullptr);
+    ClearHandleInternal();
 }
 
 void UnixThread::ClearHandleInternal()

Source/Engine/Platform/Win32/Win32Thread.cpp

@@ -113,6 +113,7 @@ unsigned long Win32Thread::ThreadProc(void* pThis)
 void Win32Thread::Join()
 {
     WaitForSingleObject((HANDLE)_thread, INFINITE);
+    ClearHandleInternal();
 }
 
 void Win32Thread::ClearHandleInternal()

Source/Engine/Threading/JobSystem.cpp

@@ -149,8 +149,7 @@ void JobSystemService::Dispose()
     {
         if (Threads[i])
         {
-            if (Threads[i]->IsRunning())
-                Threads[i]->Kill(true);
+            Threads[i]->Kill(true);
             Delete(Threads[i]);
             Threads[i] = nullptr;
         }

Source/Engine/Threading/ThreadPool.cpp

@@ -98,10 +98,7 @@ void ThreadPoolService::Dispose()
     // Delete threads
     for (int32 i = 0; i < ThreadPoolImpl::Threads.Count(); i++)
     {
-        if (ThreadPoolImpl::Threads[i]->IsRunning())
-        {
-            ThreadPoolImpl::Threads[i]->Kill(true);
-        }
+        ThreadPoolImpl::Threads[i]->Kill(true);
     }
     ThreadPoolImpl::Threads.ClearDelete();
 }