Do not rely on route attributes being present

Do not rely on route attributes being present

Fixes the following exception:
java.lang.NullPointerException: null
        at org.opendaylight.protocol.bgp.rib.impl.RIBImpl.updateTables(RIBImpl.java:126) ~[na:na]
        at org.opendaylight.protocol.bgp.rib.impl.BGPPeer.onMessage(BGPPeer.java:46) ~[na:na]
        at org.opendaylight.protocol.bgp.rib.impl.BGPPeer.onMessage(BGPPeer.java:32) ~[na:na]
        at org.opendaylight.protocol.bgp.rib.impl.BGPSessionImpl.handleMessage(BGPSessionImpl.java:199) ~[na:na]
        at org.opendaylight.protocol.bgp.rib.impl.BGPSessionImpl.handleMessage(BGPSessionImpl.java:50) ~[na:na]
        at org.opendaylight.protocol.framework.AbstractProtocolSession.channelRead0(AbstractProtocolSession.java:45) ~[na:na]
        at io.netty.channel.SimpleChannelInboundHandler.channelRead(SimpleChannelInboundHandler.java:98) ~[bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [bundlefile:na]
        at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:173) [bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [bundlefile:na]
        at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:173) [bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:337) [bundlefile:na]
        at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:323) [bundlefile:na]
        at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:785) [bundlefile:na]
        at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:100) [bundlefile:na]
        at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:480) [bundlefile:na]
        at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:447) [bundlefile:na]
        at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:341) [bundlefile:na]
        at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:101) [bundlefile:na]
        at java.lang.Thread.run(Thread.java:744) [na:1.7.0_45]

Change-Id: Ief4bb355f3f3e4566235b2339d0593ab7307ea58
Signed-off-by: Robert Varga <rovarga@cisco.com>