Panic: Nil pointer dereference when mesh.peer is empty

[mattbostock]

Found during my local testing with version 0.5.1. It appears that if -mesh.peer is set but empty, Alertmanager will panic:

docker-compose up
Starting alertmanager_peer_one_1
Starting alertmanager_peer_two_1
Attaching to alertmanager_peer_two_1, alertmanager_peer_one_1
peer_two_1  | + cd /alertmanager
peer_two_1  | + exec /usr/local/bin/reefer -t alertmanager.yml.tmpl:alertmanager.yml /alertmanager/alertmanager -log.level=debug -web.external-url=https://alertmanager.example.com -web.listen-address=:9094 -storage.path=data/ -mesh.listen-address=:10004 -mesh.peer=peer_one:10003
peer_one_1  | + cd /alertmanager
peer_one_1  | + exec /usr/local/bin/reefer -t alertmanager.yml.tmpl:alertmanager.yml /alertmanager/alertmanager -log.level=debug -web.external-url=https://alertmanager.example.com -web.listen-address=:9093 -storage.path=data/ -mesh.listen-address=:10003 -mesh.peer=
peer_two_1  | time="2017-04-03T13:05:04Z" level=info msg="Starting alertmanager (version=0.5.1, branch=HEAD, revision=0ea1cac51e6a620ec09d053f0484b97932b5c902)" source="main.go:101"
peer_two_1  | time="2017-04-03T13:05:04Z" level=info msg="Build context (go=go1.8, user=root@33842653d6db, date=20170323-13:45:34)" source="main.go:102"
peer_one_1  | time="2017-04-03T13:05:04Z" level=info msg="Starting alertmanager (version=0.5.1, branch=HEAD, revision=0ea1cac51e6a620ec09d053f0484b97932b5c902)" source="main.go:101"
peer_one_1  | time="2017-04-03T13:05:04Z" level=info msg="Build context (go=go1.8, user=root@33842653d6db, date=20170323-13:45:34)" source="main.go:102"
peer_two_1  | time="2017-04-03T13:05:04Z" level=info msg="Loading configuration file" file=alertmanager.yml source="main.go:195"
peer_two_1  | time="2017-04-03T13:05:04Z" level=info msg="Listening on :9094" source="main.go:250"
peer_one_1  | time="2017-04-03T13:05:04Z" level=info msg="Loading configuration file" file=alertmanager.yml source="main.go:195"
peer_one_1  | panic: runtime error: invalid memory address or nil pointer dereference
peer_one_1  | [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x8092fa]
peer_one_1  |
peer_one_1  | goroutine 14 [running]:
peer_one_1  | github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh.(*connectionMaker).connectionTerminated.func1(0xc420036f40)
peer_one_1  | 	/go/src/github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh/connection_maker.go:195 +0xfa
peer_one_1  | github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh.(*connectionMaker).queryLoop(0xc420050f60, 0xc420050f00)
peer_one_1  | 	/go/src/github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh/connection_maker.go:223 +0xf6
peer_one_1  | created by github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh.newConnectionMaker
peer_one_1  | 	/go/src/github.com/prometheus/alertmanager/vendor/github.com/weaveworks/mesh/connection_maker.go:74 +0x21a
alertmanager_peer_one_1 exited with code 2