[VPP-636] VPP crashes upon deleting IPv6 addressed interface

[vvalderrv]

VPP Crashes after running the following command series:

SCRIPT: memclnt_create name vts-python
SCRIPT: want_interface_events pid 91476 enable 1
SCRIPT: want_ip4_arp_events pid 0 address 0.0.0.0
SCRIPT: want_ip6_nd_events pid 0 address 3a3a::
SCRIPT: punt protocol 17
SCRIPT: bridge_domain_add_del bd_id 1 flood 1 uu-flood 1 forward 1 learn 0 arp-term 10
SCRIPT: create_loopback mac 00:00:00:00:00:00
SCRIPT: sw_interface_add_del_address sw_if_index 1 dead:beef:c001::1/112
SCRIPT: sw_interface_dump all
SCRIPT: sw_interface_add_del_address sw_if_index 1 dead:beef:c001::1/112 del
SCRIPT: delete_loopback sw_if_index 1

Back trace:
#0 0x00007ffff5dbf418 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:54
#1 0x00007ffff5dc101a in __GI_abort () at abort.c:89
#2 0x00000000004079bd in os_panic () at /home/wdec/vpp/build-data/../src/vpp/vnet/main.c:258
#3 0x00007ffff65a3235 in debugger () at /home/wdec/vpp/build-data/../src/vppinfra/error.c:84
#4 0x00007ffff65a363d in _clib_error (how_to_die=2, function_name=0x0, line_number=0,
fmt=0x7ffff7376720 "%s:%d (%s) assertion `%s' fails")
at /home/wdec/vpp/build-data/../src/vppinfra/error.c:143
#5 0x00007ffff6f0db3a in vnet_get_sw_interface (vnm=0x6e1060 <vnet_main>, sw_if_index=1)
at /home/wdec/vpp/build-data/../src/vnet/interface_funcs.h:52
#6 0x00007ffff6f0dca5 in vnet_sw_interface_get_flags (vnm=0x6e1060 <vnet_main>, sw_if_index=1)
at /home/wdec/vpp/build-data/../src/vnet/interface_funcs.h:174
#7 0x00007ffff6f0dcf9 in vnet_sw_interface_is_admin_up (vnm=0x6e1060 <vnet_main>, sw_if_index=1)
at /home/wdec/vpp/build-data/../src/vnet/interface_funcs.h:181
#8 0x00007ffff6f17a3c in ip6_neighbor_process_timer_event (vm=0x7ffff7999340 <vlib_global_main>,
node=0x7fffb5781000, frame=0x0) at /home/wdec/vpp/build-data/../src/vnet/ip/ip6_neighbor.c:2338
#9 0x00007ffff6f17fe2 in ip6_icmp_neighbor_discovery_event_process (vm=0x7ffff7999340 <vlib_global_main>,
node=0x7fffb5781000, frame=0x0) at /home/wdec/vpp/build-data/../src/vnet/ip/ip6_neighbor.c:2468
#10 0x00007ffff76f3a73 in vlib_process_bootstrap (_a=140736241855760)
at /home/wdec/vpp/build-data/../src/vlib/main.c:1218