vcl: fix UB in Window::ImplGetFirstOverlapWindow()

mpWindowImpl can be nullptr here, see online.git's
unit-load-torture test:

vcl/source/window/window2.cxx:882:24: runtime error: member access within null pointer of type 'WindowImpl'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior vcl/source/window/window2.cxx:882:24 in

Surrouding code already checks for nullptr mpWindowImpl, so fix it directly
where the problem is reported, not a caller.

(Also fix a similar case in Window::ImplCallFocusChangeActivate().)

Change-Id: I34dee0fd49483c428a78fd48b54c00b2f0a26417
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/88474
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

vcl/source/window/event.cxx

@@ -588,7 +588,17 @@ void Window::ImplCallFocusChangeActivate( vcl::Window* pNewOverlapWindow,
     bool bCallActivate = true;
     bool bCallDeactivate = true;
 
+    if (!pOldOverlapWindow)
+    {
+        return;
+    }
+
     pOldRealWindow = pOldOverlapWindow->ImplGetWindow();
+    if (!pNewOverlapWindow)
+    {
+        return;
+    }
+
     pNewRealWindow = pNewOverlapWindow->ImplGetWindow();
     if ( (pOldRealWindow->GetType() != WindowType::FLOATINGWINDOW) ||
          pOldRealWindow->GetActivateMode() != ActivateModeFlags::NONE )

vcl/source/window/window2.cxx

@@ -879,6 +879,11 @@ vcl::Window* Window::ImplGetBorderWindow() const
 
 vcl::Window* Window::ImplGetFirstOverlapWindow()
 {
+    if (!mpWindowImpl)
+    {
+        return nullptr;
+    }
+
     if ( mpWindowImpl->mbOverlapWin )
         return this;
     else