handle destruction of unattached threads

Author: VSadov

src/coreclr/vm/ceemain.cpp

@@ -1815,6 +1815,13 @@ void OsDetachThread(void* thread)
     ASSERT(g_flsIndex != FLS_OUT_OF_INDEXES);
     void* threadFromCurrentFiber = FlsGetValue(g_flsIndex);
 
+    if (threadFromCurrentFiber == NULL)
+    {
+        // thread was never attached.
+        _ASSERTE(flsState == FLS_STATE_CLEAR);
+        return;
+    }
+
     if (threadFromCurrentFiber != thread)
     {
         _ASSERTE_ALL_BUILDS(!"Detaching a thread from the wrong fiber");