[VPP-1803] FIB crash when adjacency sourced entry cover changes

[vvalderrv]

backtrace:

#9 0x00007f87803d8090 in _clib_error (how_to_die=2, function_name=0x0, line_number=0, fmt=0x7f8781bfc870 "%s:%d (%s) assertion `%s' fails") at /home/vagrant/vpp/src/vppinfra/error.c:143
#10 0x00007f87819bf424 in fib_entry_src_adj_cover_update (src=0x7f87401692d8, fib_entry=0x7f8740164ed0) at /home/vagrant/vpp/src/vnet/fib/fib_entry_src_adj.c:381
#11 0x00007f87819b9e59 in fib_entry_src_action_cover_update (fib_entry=0x7f8740164ed0, esrc=0x7f87401692d8) at /home/vagrant/vpp/src/vnet/fib/fib_entry_src.c:237
#12 0x00007f87819b2df0 in fib_entry_cover_updated (fib_entry_index=47) at /home/vagrant/vpp/src/vnet/fib/fib_entry.c:1368
#13 0x00007f87819bfdcf in fib_entry_cover_update_one (cover=0x7f8740164198, covered=47, args=0x0) at /home/vagrant/vpp/src/vnet/fib/fib_entry_cover.c:169
#14 0x00007f87819bfc8d in fib_entry_cover_walk_node_ptr (depend=0x7f8740164008, args=0x7f873f8c8b40) at /home/vagrant/vpp/src/vnet/fib/fib_entry_cover.c:81
#15 0x00007f87819af535 in fib_node_list_walk (list=65, fn=0x7f87819bfc52 <fib_entry_cover_walk_node_ptr>, args=0x7f873f8c8b40) at /home/vagrant/vpp/src/vnet/fib/fib_node_list.c:375
#16 0x00007f87819bfcf5 in fib_entry_cover_walk (cover=0x7f8740164198, walk=0x7f87819bfdb2 <fib_entry_cover_update_one>, args=0x0) at /home/vagrant/vpp/src/vnet/fib/fib_entry_cover.c:105
#17 0x00007f87819bfdfa in fib_entry_cover_update_notify (fib_entry=0x7f8740164198) at /home/vagrant/vpp/src/vnet/fib/fib_entry_cover.c:178
#18 0x00007f87819b1ea2 in fib_entry_post_update_actions (fib_entry=0x7f8740164198, source=FIB_SOURCE_API, old_flags=FIB_ENTRY_FLAG_DROP) at /home/vagrant/vpp/src/vnet/fib/fib_entry.c:813
#19 0x00007f87819b1f96 in fib_entry_source_change_w_flags (fib_entry=0x7f8740164198, old_source=FIB_SOURCE_DEFAULT_ROUTE, old_flags=FIB_ENTRY_FLAG_DROP, new_source=FIB_SOURCE_API) at /home/vagrant/vpp/src/vnet/fib/fib_entry.c:867
#20 0x00007f87819b2a88 in fib_entry_update (fib_entry_index=0, source=FIB_SOURCE_API, flags=FIB_ENTRY_FLAG_NONE, paths=0x7f874069b370) at /home/vagrant/vpp/src/vnet/fib/fib_entry.c:1258
#21 0x00007f878199f6ee in fib_table_entry_update (fib_index=0, prefix=0x7f873f8c8d50, source=FIB_SOURCE_API, flags=FIB_ENTRY_FLAG_NONE, paths=0x7f874069b370) at /home/vagrant/vpp/src/vnet/fib/fib_table.c:773
#22 0x00007f87819e2410 in fib_api_route_add_del (is_add=1 '\001', is_multipath=0 '\000', fib_index=0, prefix=0x7f873f8c8d50, entry_flags=FIB_ENTRY_FLAG_NONE, rpaths=0x7f874069b370) at /home/vagrant/vpp/src/vnet/fib/fib_api.c:488
#23 0x00007f87814c5fe4 in ip_route_add_del_t_handler (mp=0x1300a6f40, stats_index=0x7f873f8c8d98) at /home/vagrant/vpp/src/vnet/ip/ip_api.c:694

repro:

DBGvpp# loop cre
DBGvpp# set int state loop0 up
DBGvpp# set int ip addr loop0 127.0.0.1/32
DBGvpp# set ip arp loop0 127.0.0.1 00:11:22:33:44:55
DBGvpp# ip route 0.0.0.0/0 via 127.0.0.1 loop0