Attempt to fix the _refreshSizeUnderLock crash (#13857)

See #12176 (comment), and MSFT:39723014.

I have literally no idea how to repro this one, or debug it. The dump I looked at looked like there was a `SwapChainScaleChanged` that was being dispatched as the app was tearing down. The `ControlCore` had started closing, but the `TermControl` hadn't yet. Apparently, just none of the `_refreshSizeUnderLock` callers checked if we were already closing.

All the callers appear to be on the main thread.

Closes #12176

Since there's no real way for me to repro this manually, I'm thinking we fire this fix off to the OS terminal build, where we'll pretty quickly be able to see if this fixed it or not.

src/cascadia/TerminalControl/ControlCore.cpp

@@ -911,6 +911,11 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     // - <none>
     void ControlCore::_refreshSizeUnderLock()
     {
+        if (_IsClosing())
+        {
+            return;
+        }
+
         auto cx = gsl::narrow_cast<til::CoordType>(_panelWidth * _compositionScale);
         auto cy = gsl::narrow_cast<til::CoordType>(_panelHeight * _compositionScale);