ip6-nd: fix coverity warning
ip6-nd: fix coverity warning

Restructure code to avoid NULL dereference.

Type: fix
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>
Change-Id: If3a4319f1b93af272b7b315a9b15ba4ee1f8e7ae

FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp