If all rbuf slots are in use, `_rbuf_gc` removes the oldest entry even if the
entry for the current fragment exists. This effectively decreases usable slots
by one. This patch makes `_rbuf_gc` removes the oldest entry only if there is
no entry for the current fragment.
https://tools.ietf.org/html/rfc4944#section-5.3 says:
> If a link fragment that overlaps another fragment is received, as
> identified above, and differs in either the size or datagram_offset
> of the overlapped fragment, the fragment(s) already accumulated in
> the reassembly buffer SHALL be discarded. A fresh reassembly may be
> commenced with the most recently received link fragment.
