I got this panic in a test:
```
=== RUN TestJetStreamClusterAccountLoadFailure
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x78 pc=0xb1501b]
goroutine 47853 [running]:
github.com/nats-io/nats-server/v2/server.(*jetStream).processLeaderChange(0xc000b60580, 0x0)
/home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:3638 +0x9b
github.com/nats-io/nats-server/v2/server.(*jetStream).monitorCluster(0xc000b60580)
/home/travis/gopath/src/github.com/nats-io/nats-server/server/jetstream_cluster.go:853 +0x60f
created by github.com/nats-io/nats-server/v2/server.(*Server).startGoRoutine
/home/travis/gopath/src/github.com/nats-io/nats-server/server/server.go:3017 +0x87
FAIL github.com/nats-io/nats-server/v2/server 227.888s
```
which from that branch would point to function processLeaderChange()
line:
```
} else if node := js.getMetaGroup().GroupLeader(); node == _EMPTY_ {
```
which I guess meant that getMetaGroup() was returning `nil`.
Refactored a bit to get the group leader in 2 steps.
Signed-off-by: Ivan Kozlovic <ivan@synadia.com>
e75020e275