ACPICA: Fix possible memory leak in nspredef

Fixed a possible leak when an attempt is made to repair a return
object.  The only current repair is an automatic buffer to string
conversion.

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>