Don't try to pop from an empty stack in wxGDIPlusContext::PopState().

Using PopState() without a previous PushState() is an error but it shouldn't
result in a crash, so add an assert guarding against it.

Closes #13197.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68344 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/graphics.cpp

@@ -1547,6 +1547,8 @@ void wxGDIPlusContext::PushState()
 
 void wxGDIPlusContext::PopState()
 {
+    wxCHECK_RET( !m_stateStack.empty(), wxT("No state to pop") );
+
     GraphicsState state = m_stateStack.top();
     m_stateStack.pop();
     m_context->Restore(state);