Avoid selecting all rows up to UINT_MAX in generic wxDataViewCtrl.

Shift clicking in a control with multiple selections without a previous
current row attempted to select all rows from the current one up to UINT_MAX
which resulted in a program freezing (and probably running out of memory in 64
bit builds).

Fix this by explicitly checking for the absence of the current item.

Closes #16582.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77941 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix wxMSW build in UTF-8 build.

Need an explicit cast to convert the buffer returned by wxString::t_str() to a
pointer in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77942 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775