fix panic in RegisterHealthCheck when stopping serviced

ISSUE - seen when stopping serviced with services started:
```
E0912 03:37:29.519999 09683 runningservice.go:91] Unable to get service 75n1qbru6n7scp94yyr5y1xt8: Get http://localhost:9200/controlplane/service/9wfvgqylgygmxojur0r6d063: EOF
panic: runtime error: index out of range

goroutine 12637 [running]:
runtime.panic(0xab83c0, 0x111fe7c)
	/usr/local/go/src/pkg/runtime/panic.c:279 +0xf5
github.com/control-center/serviced/health.RegisterHealthCheck(0xc209376f20, 0x19, 0xc209abc730, 0xf, 0xc209abc740, 0x6, 0x7f190f9335d0, 0xc208212630)
	/home/plu/src/europa/src/golang/src/github.com/control-center/serviced/health/health.go:101 +0x807
github.com/control-center/serviced/dao/elasticsearch.(*ControlPlaneDao).LogHealthCheck(0xc208212630, 0xc209376f20, 0x19, 0xc209abc730, 0xf, 0xc209c86840, 0x27, 0xc209abc740, 0x6, 0xc209abc748, ...)
	/home/plu/src/europa/src/golang/src/github.com/control-center/serviced/dao/elasticsearch/health.go:26 +0xe5
```