Fix a mismerge from NetBSD in r162194 with xdr_rpcb_entry_list_ptr(..)

This fixes the potential NULL pointer dereference properly, and also fixes
memory leaks encountered in the process of iterating through *rp.

MFC after: 1 week
Found by: Valgrind
Reported by: Dan Roberts <dan.roberts@isilon.com>
Submitted by: Miles Ohlrich <miles.ohlrich@isilon.com>
Sponsored by: EMC / Isilon Storage Division