fix assert when the control doesn't have a valid date initially (bug 1648192)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@44347 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/datectrl.cpp

@@ -259,7 +259,8 @@ void wxDatePickerCtrl::SetValue(const wxDateTime& dt)
     // we need to keep only the date part, times don't make sense for this
     // control (in particular, comparisons with other dates would fail)
     m_date = dt;
-    m_date.ResetTime();
+    if ( m_date.IsValid() )
+        m_date.ResetTime();
 }
 
 wxDateTime wxDatePickerCtrl::GetValue() const