Fix topology not showing prefixes learned from BGP/LS

The Linkstate RIB plugin was losing IP reachability information in the
prefix case. Fix it by using the copy builder constructor.

Also add a defensive check for IP reachability presence in the topology
builder.

Change-Id: I0b16b036397a935771d86a3c7253c998e594ec7c
Signed-off-by: Robert Varga <rovarga@cisco.com>