Fix incorrect assertions and possible use-after-free in evrpc_free()

Fix incorrect assertions and possible use-after-free in evrpc_free()

Original description:

  The following patch fixes incorrect assertions in evrpc_free():
  evrpc_unregister_rpc() and evrpc_remove_hook() return 0 for success.

  Also, in evrpc_unregister_rpc(), it is better to free RPC structure
  at the end: evrpc_free() uses rpc->uri as "name" parameter when
  calling evrpc_unregister_rpc(), then rpc->uri is freed, but we have
  "registered_uri = evrpc_construct_uri(name)". So at this time "name"
  is invalid.