bgpd: fix, do not access peer->notify.data when it is null

bgpd: fix, do not access peer->notify.data when it is null

bgp crash on rt2 spotted when 2 commands
launched consequently:
T0: rr, config -> router bgp 65004 -> neighbor 192.168.12.2 password
8888
T1: rt2, snmpwalk -v 2c -c public 127.0.0.1 .1.3.6.1.4.1.7336.4.2.1
T2: rt2 bgp is crashed.

config rt2-bgp:
---------------

!
debug bgp updates
!
router bgp 65002
 no bgp ebgp-requires-policy
 no bgp network import-check
 no bgp default ipv4-unicast
 neighbor 192.168.12.4 remote-as external
 neighbor 192.168.12.4 timers 1 3
 neighbor 192.168.12.4 timers connect 1
 neighbor 2001:db8::12:4 remote-as external
 neighbor 2001:db8::12:4 timers 1 3
 neighbor 2001:db8::12:4 timers connect 1
 !
 address-family ipv4 unicast
  neighbor 192.168.12.4 activate
  neighbor 192.168.12.4 addpath-tx-all-paths

 exit-address-family
 address-family ipv6 unicast
  neighbor 2001:db8::12:4 activate
 exit-address-family
!
agentx
!

config rt2-snmpd:
agentAddress 127.0.0.1,[::1]

group public_group v1 public
group public_group v2c public
access public_group "" any noauth prefix all all none

rocommunity public default

view all included .1

iquerySecName frr
rouser frr

master agentx

agentXSocket /etc/frr/agentx
agentXPerms 777 755 root frr

config rt2-zebra:
-----------------

!
interface r2-eth0
 ip address 192.168.12.2/24
 ipv6 address 2001:db8::12:2/64
!

config rr-bgpd:
---------------

!
debug bgp updates
!
router bgp 65004
 no bgp ebgp-requires-policy
 no bgp network import-check
 no bgp default ipv4-unicast
 neighbor 192.168.12.1 remote-as external
 neighbor 192.168.12.1 timers 1 3
 neighbor 192.168.12.1 timers connect 1
 neighbor 192.168.12.2 remote-as external
 neighbor 192.168.12.2 timers 1 3
 neighbor 192.168.12.2 timers connect 1
 neighbor 192.168.12.3 remote-as external
 neighbor 192.168.12.3 timers 1 3
 neighbor 192.168.12.3 timers connect 1
 neighbor 2001:db8::12:1 remote-as external
 neighbor 2001:db8::12:1 timers 1 3
 neighbor 2001:db8::12:1 timers connect 1
 neighbor 2001:db8::12:2 remote-as external
 neighbor 2001:db8::12:2 timers 1 3
 neighbor 2001:db8::12:2 timers connect 1
 neighbor 2001:db8::12:3 remote-as external
 neighbor 2001:db8::12:3 timers 1 3
 neighbor 2001:db8::12:3 timers connect 1
 !
 address-family ipv4 unicast
  neighbor 192.168.12.1 activate
  neighbor 192.168.12.1 addpath-tx-all-paths
  neighbor 192.168.12.1 route-server-client
  neighbor 192.168.12.2 activate
  neighbor 192.168.12.2 addpath-tx-all-paths
  neighbor 192.168.12.2 route-server-client
  neighbor 192.168.12.3 activate
  neighbor 192.168.12.3 addpath-tx-all-paths
  neighbor 192.168.12.3 route-server-client
 exit-address-family
 address-family ipv6 unicast
  neighbor 2001:db8::12:1 activate
  neighbor 2001:db8::12:1 route-server-client
  neighbor 2001:db8::12:2 activate
  neighbor 2001:db8::12:2 route-server-client
  neighbor 2001:db8::12:3 activate
  neighbor 2001:db8::12:3 route-server-client
 exit-address-family
!
agentx
!

config rr-zebra:
----------------
!
interface rr-eth0
 ip address 192.168.12.4/24
 ipv6 address 2001:db8::12:4/64
!

Fixes: 2d8fff6b81bb ("bgpd: Implement BGP4V2-MIB(bgp4V2PeerErrorsTable)")
Signed-off-by: Dmytro Shytyi <dmytro.shytyi@6wind.com>