Fix crash when reparenting the focused window to another TLW

If the window stored as m_winLastFocused in one TLW was reparented to
another one and then destroyed, this pointer to it wasn't updated and
became dangling.

Fix this by using a safe weak reference instead of raw pointer for
m_winLastFocused. This ensures that it can never be used when it becomes
invalid.

Closes #17980.

src/msw/toplevel.cpp

@@ -102,8 +102,6 @@ void wxTopLevelWindowMSW::Init()
     m_fsIsMaximized = false;
     m_fsIsShowing = false;
 
-    m_winLastFocused = NULL;
-
     m_menuSystem = NULL;
 }
 
@@ -1195,7 +1193,9 @@ void wxTopLevelWindowMSW::DoRestoreLastFocus()
         parent = this;
     }
 
-    wxSetFocusToChild(parent, &m_winLastFocused);
+    wxWindow* winPtr = m_winLastFocused;
+    wxSetFocusToChild(parent, &winPtr);
+    m_winLastFocused = winPtr;
 }
 
 void wxTopLevelWindowMSW::OnActivate(wxActivateEvent& event)