bgpd: Do not try to uninstall BFD session if the peer is not established

bgpd: Do not try to uninstall BFD session if the peer is not established

Having something like:

```
 neighbor 192.168.1.222 ebgp-multihop 32
 neighbor 192.168.1.222 update-source 192.168.1.5
 neighbor 192.168.1.222 bfd
```

Won't work and the result is (empty):

```
$ show bfd peers
BFD Peers:
```

bgp_stop() is called in BGP FSM multiple times (even at startup) that
causes intermediate session interruption when update-source/ebgp-multihop
is triggered.

With this fix, the ordering does not matter and the BFD session's parameters
are updated correctly.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>