Corrected test for region validity in wxMSW wxRegion::DoOffset().

Corrected test for region validity in wxMSW wxRegion::DoOffset().

Checking M_REGION is not enough as the region can be invalid (meaning
m_refData dereferenced inside M_REGION is NULL itself) and not just not
initialized, so it resulted in crashes and not just the expected assert
failure when wxRegion::Offset() was called for an invalid region.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70017 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775