reset m_selectionOld when the selection is programatically changed

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31035 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/combobox.cpp

@@ -251,6 +251,7 @@ WXLRESULT wxComboBox::MSWWindowProc(WXUINT nMsg, WXWPARAM wParam, WXLPARAM lPara
                 m_value = wxEmptyString;
             else
                 m_value = GetString(wParam);
+            m_selectionOld = -1;
             break;
     }