Fix bug with using uninitialized flags in GetParentForModalDialog().

Fix bug with using uninitialized flags in GetParentForModalDialog().

GetParentForModalDialog() was called from the ctor initialized list before
m_windowStyle could be initialized by the base class ctor in several different
places, meaning that the check for wxDIALOG_NO_PARENT in this function was
using uninitialized variable.

Fix this by passing the style parameter explicitly to this function to allow
using it from derived class ctors. Still keep an overload which uses the
actual window parent and flags which is simpler to use for later calls to this
function.

Thanks valgrind for finding this one.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64019 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775