message: Fix DrawRoundedRectangle assert with wxGTK 3.0
  Don't draw if w=0.
  Fixes
diff: @@ -251,7 +251,8 @@ private:
  gc->SetPen(*wxTRANSPARENT_PEN);
  auto rect = GetClientRect();
  - gc->DrawRoundedRectangle(rect.x, rect.y, rect.width, rect.height, PX(2));
  + if (!rect.IsEmpty())
  + gc->DrawRectangle(rect.x, rect.y, rect.width, rect.height);
  }
  wxColour m_fg, m_bg;
  