Don't throw NPE if we fail to find ChildOf interface

Don't throw NPE if we fail to find ChildOf interface

Check for null return, this NPE can happen in the following path:

2014-05-12 06:56:56.740 PDT [http-bio-8080-exec-3] WARN  o.o.y.s.b.g.i.LazyGeneratedCodecRegistry - Exception during preparation of instance identifier codec for  path InstanceIdentifier{targetType=interface org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.rsvp.rev130820.basic.explicit.route.subobjects.subobject.type.ip.prefix._case.IpPrefix, path=[org.opendaylight.yang.gen.v1.urn.tbd.params.xml.ns.yang.network.topology.rev131021.NetworkTopology, org.opendaylight.yang.gen.v1.urn.tbd.params.xml.ns.yang.network.topology.rev131021.network.topology.Topology, org.opendaylight.yang.gen.v1.urn.tbd.params.xml.ns.yang.network.topology.rev131021.network.topology.topology.Node, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.topology.pcep.rev131024.Node1, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.topology.pcep.rev131024.pcep.client.attributes.PathComputationClient, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.topology.pcep.rev131024.pcep.client.attributes.path.computation.client.ReportedLsp, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.topology.pcep.rev131024.pcep.client.attributes.path.computation.client.reported.lsp.Path, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.pcep.types.rev131005.explicit.route.object.Ero, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.pcep.types.rev131005.explicit.route.object.ero.Subobject, org.opendaylight.yang.gen.v1.urn.opendaylight.params.xml.ns.yang.rsvp.rev130820.basic.explicit.route.subobjects.subobject.type.ip.prefix._case.IpPrefix]}.
java.lang.NullPointerException: null
	at org.opendaylight.yangtools.yang.binding.util.BindingReflections.isAugmentationChild(BindingReflections.java:173) ~[bundlefile:na]
	at org.opendaylight.yangtools.sal.binding.generator.impl.InstanceIdentifierCodecImpl._serializePathArgument(InstanceIdentifierCodecImpl.java:165) ~[bundlefile:na]

Change-Id: I8d4b94d3c98dbf1f6cae36ab015282ae7afa098f
Signed-off-by: Robert Varga <rovarga@cisco.com>