Avoid crashes in wxGenericListCtrl client<->screen conversion code.

Avoid crashes in wxGenericListCtrl client<->screen conversion code.

At least in wxUniv build, DoScreenToClient() can be called before the main
control window is created which results in a crash when attempting to forward
to its DoScreenToClient() version.

Check for m_mainWin being non-NULL before using it to at least avoid the
crash, even if it's not really clear whether this is a 100% correct fix.

Closes #12390.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65831 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775