zebra: fix crash in vrf-vni mapping

zebra: fix crash in vrf-vni mapping

As part of PR 6758 vrf vni converted to transactional cli.
Handle a scenario where vrf is not created yet (inactive) and vni
is mapped to the inactive vrf.

Testing Done:

bharat(config-vrf)# do show vrf
vrf vrf1 id 11 table 1001
vrf vrf5 inactive (configured)

bharat(config)# vrf vrf5
bharat(config-vrf)# vni 5005

bharat(config-vrf)# do show vrf vni
VRF                                   VNI        VxLAN IF             L3-SVI               State Rmac
vrf5                                  5005       None                 None                 Down  None

bharat(config-vrf)# no vni 5005
bharat(config-vrf)# do show vrf vni
VRF                                   VNI        VxLAN IF             L3-SVI               State Rmac

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>