fib: fix adj_get crash

fib: fix adj_get crash

adj_get() crashed in worker thread. The index is valid but was freed.

0x00007fba5fb07921 in __GI_abort () at abort.c:79
0x00007fba528de7c3 in os_panic () at /development/libvpp/src/vpp/vnet/main.c:618
0x00007fba506f2a19 in debugger () at /development/libvpp/src/vppinfra/error.c:84
0x00007fba506f2797 in _clib_error (how_to_die=2, function_name=0x0, line_number=0, fmt=0x7fba521cecd9 "%s:%d (%s) assertion `%s' fails") at /development/libvpp/src/vppinfra/error.c:143
0x00007fba51d88254 in adj_get (adj_index=79) at /development/libvpp/src/vnet/adj/adj.h:472
0x00007fba51d87642 in icmp6_neighbor_solicitation_or_advertisement (vm=0x7fb9f19a0580, node=0x7fb9bb820340, frame=0x7fb9b718bc40, is_solicitation=0) at /development/libvpp/src/vnet/ip6-nd/ip6_nd.c:134
0x00007fba51d8f479 in icmp6_neighbor_advertisement (vm=0x7fb9f19a0580, node=0x7fb9bb820340, frame=0x7fb9b718bc40) at /development/libvpp/src/vnet/ip6-nd/ip6_nd.c:348

Type: fix

Signed-off-by: Rajith P R <rajith@rtbrick.com>
Change-Id: Ibe29854137ee9680f7947450eb7e256b8c0ade31