ip,ip6: fix uninitialized nexthop in "addr del" event

When an IP address is removed, the RIB cleanup process (rib4/6_cleanup)
resets associated nexthop memory before the addr del event is sent. As a
result, the event includes a nexthop structure with all fields set to zero,
even though a valid nexthop was previously in use.

To reproduce:

grcli show events
> addr add: iface[1] 9.6.6.7
> route del: 9.6.6.7/24 via 9.6.6.7 origin link
> nh del: iface 1 vrf 0 9.6.6.7 d2:f0:0c:bc:a4:10
> addr del: iface[0] 0x5d279f4eb1ac

Or from another instance:

grout# add ip address 9.6.6.7/24 iface p4
grout# del ip address 9.6.6.7/24 iface p4

Fix this by emitting the addr del event before the nexthop is cleaned up,
ensuring the event still carries valid nexthop information.

Signed-off-by: Maxime Leroy <[email protected]>