Avoid assertion failure when typing into generic wxDatePickerCtrl

Don't call wxCalendarCtrl::SetDate() if the current date is invalid,
this would just result in an assertion failure.

src/generic/datectlg.cpp

@@ -192,6 +192,7 @@ private:
         if ( (dt.IsValid() && (!dtOld.IsValid() || dt != dtOld)) ||
                 (!dt.IsValid() && dtOld.IsValid()) )
         {
+            if ( dt.IsValid() )
                 SetDate(dt);
             SendDateEvent(dt);
         }