Don't use invalid pen in wxDCImpl::DoGradientFillConcentric().

Don't use invalid pen in wxDCImpl::DoGradientFillConcentric().

The old code stored the original colour of m_pen before changing it and
restored it on exit but this didn't work if the pen was invalid to start with.

Just store the pen itself instead, this works in any case.

Closes #11235.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62143 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775