Reset dec.wrapper to nil before deleting dtor

This is so that accidental references from routines that execute as a
result do not cause segmentation fault.  (We are about to add such a
code path.)

pkg/impl/libraptorq/decoder.go

@@ -105,8 +105,8 @@ func (dec *Decoder) FreeSourceBlock(sbn uint8) {
 func (dec *Decoder) Close() (err error) {
  switch wrapped := dec.wrapped.(type) {
  case swig.BytesDecoder:
- swig.DeleteBytesDecoder(wrapped)
  dec.wrapped = nil
+ swig.DeleteBytesDecoder(wrapped)
  default:
  err = errors.New("RaptorQ decoder already closed")
  }