config: forbid nil regexp matchers

[simonpasquier]

Otherwise the program could panic.

route:
  receiver: empty

  routes:
  - match_re:
      invalid_label:
    receiver: empty

receivers:
- name: empty

level=info ts=2019-10-24T15:31:19.726Z caller=main.go:217 msg="Starting Alertmanager" version="(version=0.19.0, branch=master, revision=a75cd02786dfecd25e2469fc4df5d920e6b9c226)"
level=info ts=2019-10-24T15:31:19.726Z caller=main.go:218 build_context="(go=go1.13.1, user=simon@simon-laptop, date=20191024-15:20:11)"
level=info ts=2019-10-24T15:31:19.758Z caller=cluster.go:161 component=cluster msg="setting advertise address explicitly" addr=192.168.1.17 port=9094
level=info ts=2019-10-24T15:31:19.759Z caller=cluster.go:623 component=cluster msg="Waiting for gossip to settle..." interval=2s
level=info ts=2019-10-24T15:31:19.781Z caller=coordinator.go:119 component=configuration msg="Loading configuration file" file=config/testdata/conf.nil-match_re-route.yml
level=info ts=2019-10-24T15:31:19.781Z caller=coordinator.go:131 component=configuration msg="Completed loading of configuration file" file=config/testdata/conf.nil-match_re-route.yml
level=info ts=2019-10-24T15:31:19.783Z caller=cluster.go:632 component=cluster msg="gossip not settled but continuing anyway" polls=0 elapsed=23.742589ms
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xb32915]

goroutine 1 [running]:
regexp.(*Regexp).String(...)
        /home/simon/go/src/regexp/regexp.go:106
github.com/prometheus/alertmanager/types.NewRegexMatcher(...)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/types/match.go:100
github.com/prometheus/alertmanager/dispatch.NewRoute(0xc000300510, 0xc0001b9380, 0x0)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/dispatch/route.go:95 +0x3f5
github.com/prometheus/alertmanager/dispatch.NewRoutes(0xc0001282f8, 0x1, 0x1, 0xc0001b9380, 0x7fde30c3edb8, 0x1, 0x100)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/dispatch/route.go:115 +0x83
github.com/prometheus/alertmanager/dispatch.NewRoute(0xc000300480, 0x0, 0xc0003ff6d0)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/dispatch/route.go:106 +0x671
github.com/prometheus/alertmanager/api/v1.(*API).Update(0xc0003c6b00, 0xc0001095e0)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/api/v1/api.go:140 +0x97
github.com/prometheus/alertmanager/api.(*API).Update(0xc00001df00, 0xc0001095e0, 0xc0003aa020)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/api/api.go:200 +0x38
main.run.func7(0xc0001095e0, 0x8, 0x40)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/cmd/alertmanager/main.go:412 +0x596
github.com/prometheus/alertmanager/config.(*Coordinator).notifySubscribers(0xc00052bb00, 0xc00001a940, 0x4)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/config/coordinator.go:88 +0x72
github.com/prometheus/alertmanager/config.(*Coordinator).Reload(0xc00052bb00, 0x0, 0x0)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/config/coordinator.go:136 +0x5ab
main.run(0x0)
        /home/simon/workspace/src/github.com/prometheus/alertmanager/cmd/alertmanager/main.go:440 +0x3f95
main.main()
        /home/simon/workspace/src/github.com/prometheus/alertmanager/cmd/alertmanager/main.go:173 +0x22