Fix bug when copying slice to itself

If proptools.CopyProperties is passed two values that point same
slice then setting the destination slice to a new slice will
overwrite the source slice, and the properties struct that is both
the source and destination will have an empty slice.  Copy into
the new slice using a new reflect.Value, and then update the
destination.

Change-Id: I1bfcdc51e4278ea7c7ed81dafc928a5471219f05