bgpd: Fix `dampening flap-statistics json` crash

bgpd: Fix `dampening flap-statistics json` crash

With fix:
```
exit1-debian-9# sh ip bgp dampening flap-statistics
BGP table version is 22, local router ID is 10.10.10.200, vrf id 0
Default local pref 100, local AS 65001
Status codes:  s suppressed, d damped, h history, * valid, > best, = multipath,
               i internal, r RIB-failure, S Stale, R Removed
Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
Origin codes:  i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

   Network          From            Flaps Duration Reuse    Path
*d 10.0.0.0/24      192.168.0.2     4    00:00:50 00:35:56 65000 ?
*d 10.10.10.100/32  192.168.0.2     4    00:00:50 00:35:56 65000 ?
*d 192.168.1.0/24   192.168.0.2     4    00:00:50 00:35:56 65000 ?

Displayed  3 routes and 10 total paths
exit1-debian-9# sh ip bgp dampening flap-statistics json
{
 "vrfId": 0,
 "vrfName": "default",
 "tableVersion": 22,
 "routerId": "10.10.10.200",
 "defaultLocPrf": 100,
 "localAS": 65001,
 "routes": { "10.0.0.0/24": [
  {
    "valid":true,
    "damped":true,
    "pathFrom":"external",
    "peerHost":"192.168.0.2",
    "bdiFlap":4,
    "peerUptime":"00:00:54",
    "peerUptimeMsec":54000,
    "peerUptimeEstablishedEpoch":1626355135,
    "reuseTimerMsecs":2151000,
    "asPath":"65000",
    "origin":"?"
  }
],"10.10.10.100/32": [
  {
    "valid":true,
    "damped":true,
    "pathFrom":"external",
    "peerHost":"192.168.0.2",
    "bdiFlap":4,
    "peerUptime":"00:00:54",
    "peerUptimeMsec":54000,
    "peerUptimeEstablishedEpoch":1626355135,
    "reuseTimerMsecs":2151000,
    "asPath":"65000",
    "origin":"?"
  }
],"192.168.1.0/24": [
  {
    "valid":true,
    "damped":true,
    "pathFrom":"external",
    "peerHost":"192.168.0.2",
    "bdiFlap":4,
    "peerUptime":"00:00:54",
    "peerUptimeMsec":54000,
    "peerUptimeEstablishedEpoch":1626355135,
    "reuseTimerMsecs":2151000,
    "asPath":"65000",
    "origin":"?"
  }
] }  }
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>