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/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()