When there are more than one NLRI batched in one BGP UPDATE message, the NLRI length byte cannot be parsed correctly.

The NLRI length byte should be read for every NLRI. However, right now it's only read for the first NLRI.

Caused by: org.opendaylight.protocol.bgp.parser.BGPDocumentedException: Could not parse BGP attributes.
at org.opendaylight.protocol.bgp.parser.impl.message.BGPUpdateMessageParser.parseMessageBody(BGPUpdateMessageParser.java:135)[349:org.opendaylight.bgpcep.bgp-parser-impl:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.impl.message.BGPUpdateMessageParser.parseMessageBody(BGPUpdateMessageParser.java:46)[349:org.opendaylight.bgpcep.bgp-parser-impl:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.spi.pojo.SimpleMessageRegistry.parseBody(SimpleMessageRegistry.java:31)[348:org.opendaylight.bgpcep.bgp-parser-spi:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.spi.AbstractMessageRegistry.parseMessage(AbstractMessageRegistry.java:66)[348:org.opendaylight.bgpcep.bgp-parser-spi:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.rib.impl.BGPByteToMessageDecoder.decode(BGPByteToMessageDecoder.java:49)[361:org.opendaylight.bgpcep.bgp-rib-impl:0.6.1.Boron-SR1]
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:411)[114:io.netty.codec:4.0.37.Final]
... 21 more
Caused by: java.lang.IllegalStateException: NLRI length read from message doesn't match. Length expected (read from NLRI) is 1537, length readable is 1296
at com.google.common.base.Preconditions.checkState(Preconditions.java:197)[39:com.google.guava:18.0.0]
at org.opendaylight.protocol.bgp.flowspec.AbstractFlowspecNlriParser.verifyNlriLength(AbstractFlowspecNlriParser.java:608)[362:org.opendaylight.bgpcep.bgp-flowspec:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.flowspec.AbstractFlowspecNlriParser.parseNlri(AbstractFlowspecNlriParser.java:629)[362:org.opendaylight.bgpcep.bgp-flowspec:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.spi.pojo.SimpleNlriRegistry.parseMpReach(SimpleNlriRegistry.java:239)[348:org.opendaylight.bgpcep.bgp-parser-spi:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.impl.message.update.MPReachAttributeParser.parseAttribute(MPReachAttributeParser.java:49)[349:org.opendaylight.bgpcep.bgp-parser-impl:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.spi.pojo.SimpleAttributeRegistry.parseAttributes(SimpleAttributeRegistry.java:138)[348:org.opendaylight.bgpcep.bgp-parser-spi:0.6.1.Boron-SR1]
at org.opendaylight.protocol.bgp.parser.impl.message.BGPUpdateMessageParser.parseMessageBody(BGPUpdateMessageParser.java:131)[349:org.opendaylight.bgpcep.bgp-parser-impl:0.6.1.Boron-SR1]