Test for wxPen validity before testing for its style in wxGTK wxDC.

Test for wxPen validity before testing for its style in wxGTK wxDC.

Calling wxPen::GetStyle() on an invalid pen resulted in an assert, breaking
the grid column drawing while reordering them, for example, because the code
in wxGrid uses wxDC::DrawRectangle() after calling SetPen(wxNullPen).

Fix this by testing for the pen validity first.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64474 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775