zebra: ignore zero_mac without VNI deletes

zebra: ignore zero_mac without VNI deletes

Ignore zebra_mac updates if they do not contain a VNI for vxlan
interface. We don't have anything we can do with them.

'''
==443593== Process terminating with default action of signal 6 (SIGABRT): dumping core
==443593==    at 0x4E1156C: __pthread_kill_implementation (in /usr/lib64/libc.so.6)
==443593==    by 0x4DC4D15: raise (in /usr/lib64/libc.so.6)
==443593==    by 0x49823C7: core_handler (sigevent.c:261)
==443593==    by 0x4DC4DBF: ??? (in /usr/lib64/libc.so.6)
==443593==    by 0x4E1156B: __pthread_kill_implementation (in /usr/lib64/libc.so.6)
==443593==    by 0x4DC4D15: raise (in /usr/lib64/libc.so.6)
==443593==    by 0x4D987F2: abort (in /usr/lib64/libc.so.6)
==443593==    by 0x49C3064: _zlog_assert_failed (zlog.c:700)
==443593==    by 0x4F5E6D: zebra_vxlan_if_vni_find (zebra_vxlan_if.c:661)
==443593==    by 0x4EEAC3: zebra_vxlan_check_readd_vtep (zebra_vxlan.c:4244)
==443593==    by 0x450967: netlink_macfdb_change (rt_netlink.c:3722)
==443593==    by 0x450011: netlink_neigh_change (rt_netlink.c:4458)
'''

Signed-off-by: Stephen Worley <sworley@nvidia.com>