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