server: Fix invalid pointer dereference in updateNeighbor

[rastislavs]

Fixes panic in updateNeighbor when when timers are being updated:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0xaa8bf4]

goroutine 21 [running]:
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).updateNeighbor(0x40002da008, 0x40002ee008)
	go/src/github.com/gobgp/pkg/server/server.go:3437 +0x644
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).UpdatePeer.func1()
	go/src/github.com/gobgp/pkg/server/server.go:3465 +0x58
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).handleMGMTOp(0xc23f912b666d455d?, 0x40002eaf00)
	go/src/github.com/gobgp/pkg/server/server.go:236 +0x94
github.com/osrg/gobgp/v4/pkg/server.(*BgpServer).Serve(0x40002da008)
	§go/src/github.com/gobgp/pkg/server/server.go:381 +0x400

Also adds a unit-test exercising the updateNeighbor code path, capable of catching this (before the fix).

[fujita]

Thanks a lot, merged. Embarrassed, it was such a silly mistake.

So you guys use the feature to update peer config?

[rastislavs]

So you guys use the feature to update peer config?

Yeah we do use peer config update in Cilium.