do not dereference nil pointer when FindChildService returns nil child


ISSUE - occurring when starting services:
```
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x50c583]

goroutine 4800 [running]:
runtime.panic(0x9d0140, 0x14b8ae8)
        /usr/local/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/control-center/serviced/dao/elasticsearch.(*ControlPlaneDao).FindChildService(0xc210176990, 0xc2123a21c0, 0x19, 0xc212095138, 0x5, ...)
        /go/src/github.com/control-center/serviced/dao/elasticsearch/service.go:70 +0xc3
reflect.Value.call(0x975060, 0xa84560, 0x130, 0xa8d270, 0x4, ...)
        /usr/local/go/src/pkg/reflect/value.go:474 +0xe0b
reflect.Value.Call(0x975060, 0xa84560, 0x130, 0x7f50685cbee8, 0x3, ...)
        /usr/local/go/src/pkg/reflect/value.go:345 +0x9d
net/rpc.(*service).call(0xc210166440, 0xc2100a5100, 0xc212c0d958, 0xc2102bd580, 0xc2128bb240, ...)
        /usr/local/go/src/pkg/net/rpc/server.go:381 +0x159
created by net/rpc.(*Server).ServeCodec
        /usr/local/go/src/pkg/net/rpc/server.go:452 +0x3bb
```

DEMO:
```
W0917 15:36:37.918653 14137 service.go:78] unable to find child of service: &{ID: Name: Title: Version: Context:map[] Startup: Description: Tags:[] OriginalConfigs:map[] ConfigFiles:map[] Instances:0 InstanceLimits:{Min:0 Max:0 Default:0} ChangeOptions:[] ImageID: PoolID: DesiredState:0 HostPolicy: Hostname: Privileged:false Launch: Endpoints:[] Tasks:[] ParentServiceID: Volumes:[] CreatedAt:0001-01-01 00:00:00 +0000 UTC UpdatedAt:0001-01-01 00:00:00 +0000 UTC DeploymentID: DisableImage:false LogConfigs:[] Snapshot:{Pause: Resume:} Runs:map[] RAMCommitment:0 CPUCommitment:0 Actions:map[] HealthChecks:map[] Prereqs:[] MonitoringProfile:{MetricConfigs:[] GraphConfigs:[] ThresholdConfigs:[]} MemoryLimit:0 CPUShares:0 PIDFile: VersionedEntity:{DatabaseVersion:0}}
```

DEMO2 - services started:
```
root@ip-10-111-2-211:/var/log/upstart# serviced service status
NAME			ID				STATUS		UPTIME			HOST			IN_SYNC		DOCKER_ID
└─Zenoss.resmgr		5bxjeiep5clvb7wy0mm24bg1k	Running		8m46.286608124s		ip-10-111-2-128		Y		632ae05788a6
  ├─zeneventserver	16jq6rcq8snlke7tgke9abuje	Running		8m38.892081708s		ip-10-111-2-128		Y		af63e59b8b2f
  ├─Zauth		1ikh0bt5ycwu52ipamecka6ce	Running		9m28.022283633s		ip-10-111-2-211		Y		31503219748c
  ├─localhost		23w6jp9b0hpplnqpuq78x2vqs											
  │ ├─localhost		au47a6whlsk3p7gfmzbi3jw82	Running		8m46.425168317s		ip-10-111-2-128		Y		2ce4842a6ce3
  │ │ ├─zenwebtx	15lnilpohrygogtodzfk95p95	Running		9m30.725255901s		ip-10-111-2-33		Y		e7c0a6e24eaa
  │ │ ├─zenprocess	2fopfowfw4bo9bxriqtldhagb	Running		9m30.58164172s		ip-10-111-2-33		Y		e47f6cb073cd
  │ │ ├─zenstatus	2pey88p7d4vpiweq87uluyfpv	Running		8m46.772923078s		ip-10-111-2-211		Y		ed9ba5389597
  │ │ ├─MetricShipper	3lva9rgfjh8lybfb2rbp2yovn	Running		7m27.019378667s		ip-10-111-2-33		Y		c65bbd82d799
  │ │ ├─zenucsevents	494sc6yw0pj8pfzwfkvhqd7ai	Running		8m46.565686123s		ip-10-111-2-128		Y		a8d6e02671ef
  │ │ ├─zencommand	4aqrttqsabckwcvsrdadikfyt	Running		8m42.233992361s		ip-10-111-2-128		Y		d1123ef7e665
  │ │ ├─zenjmx		6lzpgfy08besa8roz1dx2qydu	Running		8m46.502314254s		ip-10-111-2-128		Y		a0b8eb1d7983
  │ │ ├─zminion		77vgf3d0mmiga8g0fj0rua54u	Running		8m57.043509498s		ip-10-111-2-33		Y		d5b247c493d1
  │ │ ├─zenperfsnmp	7ko60s5yonfn4cco7qr4ox6m5	Running		7m26.984277845s		ip-10-111-2-33		Y		118c7120ac5a
  │ │ ├─zentrap		8uewnb8jegahf0euszrmv28e6	Running		8m59.133921547s		ip-10-111-2-211		Y		825500d06dd3
  │ │ ├─collectorredis	a7t1c40g60z0od6uexljhisx0	Running		9m30.582536756s		ip-10-111-2-33		Y		838f5385eefd
  │ │ ├─zensyslog	afc6odp6d9lub6ch1qlrumnt9	Running		8m46.99602865s		ip-10-111-2-211		Y		fa436d7056ee
  │ │ ├─zenping		c5as4yb2mjz0i9lc5gb5vkm93	Running		9m35.468582196s		ip-10-111-2-211		Y		4b050b43d7a5
  │ │ ├─zenmailtx	cimd6cf1xq659dvg0p57chkbx	Running		9m35.691310531s		ip-10-111-2-211		Y		9fe835abbb30
  │ │ ├─zenvsphere	cqbkcmpb4ols0ms2x12cfc6xo	Running		8m46.514537305s		ip-10-111-2-128		Y		712bd664dcae
  │ │ ├─zenmodeler	mih70k01iaw3yrhxn8ghdzr		Running		8m46.43832487s		ip-10-111-2-128		Y		9540cd7092f9
  │ │ └─zenpython	ytzcastpx98897lmpgpji74r	Running		8m46.484406716s		ip-10-111-2-128		Y		408d497d3f4c
  │ └─zenhub		k3f4sakqvvosw6ygr7enan0c	Running		9m30.750470774s		ip-10-111-2-33		Y		077e13b927c7
  ├─opentsdb		2acgip4soq3gntucferu6sbas											
  │ ├─writer		8v3c4a3vckck8ukx2cbpaii3v	Running		5m21.078625212s		ip-10-111-2-33		Y		28bda7d03803
  │ └─reader		cxruejf3x0wnf2dc8t0vrcggc	Running		9m22.004051346s		ip-10-111-2-211		Y		fe2dfa58225a
  ├─MetricConsumer	3du4sbkj80ykli1qxfacbqjr8	Running		9m31.144746987s		ip-10-111-2-33		Y		3e4a10872f99
  ├─zenjserver		5l4tpw0yie1wwnecmcs050y7w	Running		8m46.945122366s		ip-10-111-2-128		Y		6ed91b135f8f
  ├─MetricShipper	5nb3uyk9eao8lodap0apij2ai	Running		9m31.012644829s		ip-10-111-2-33		Y		07f05cc6e21f
  ├─CentralQuery	5ppqo725s4923x5jyzgey4owb	Running		9m23.180265497s		ip-10-111-2-33		Y		181a5296a928
  ├─zenactiond		6cedyyoiq47ipnhd3y7ygm0yr	Running		9m35.342702506s		ip-10-111-2-211		Y		17c5cf3cf2aa
  ├─RabbitMQ		82oar1jt7qu9ygs4adcc24sr3	Running		9m9.450578255s		ip-10-111-2-33		Y		649c49511ee5
  ├─zencatalogservice	85jupiirhkdgolqr862pv5xq0	Running		8m49.777134605s		ip-10-111-2-211		Y		9003799e3c9b
  ├─HBase		8g83256hm773k08bqyvnslt9p											
  │ ├─RegionServer/0	32gpcgkr0rgsqkuytdzgxkegr/0	Running		9m11.813842851s		ip-10-111-2-211		Y		ae1e27778a4d
  │ ├─RegionServer/1	32gpcgkr0rgsqkuytdzgxkegr/1	Running		6m47.44273142s		ip-10-111-2-128		Y		dc4cba5e3f92
  │ ├─RegionServer/2	32gpcgkr0rgsqkuytdzgxkegr/2	Running		7m9.907925317s		ip-10-111-2-33		Y		73fc46ab9e66
  │ ├─HMaster		6azljvji312owrbgdk1p0zlwc	Running		7m21.645170278s		ip-10-111-2-33		Y		63cc7b59c0e9
  │ ├─ZooKeeper/0	dqu15zb0oldkt1zn4q61x8ht4/0	Running		7m15.328778228s		ip-10-111-2-33		Y		daa690f7b1f9
  │ ├─ZooKeeper/1	dqu15zb0oldkt1zn4q61x8ht4/1	Running		9m21.858657285s		ip-10-111-2-211		Y		eacc7990d245
  │ └─ZooKeeper/2	dqu15zb0oldkt1zn4q61x8ht4/2	Running		6m53.078947315s		ip-10-111-2-128		Y		3b6b21dc66bb
  ├─redis		8vdnbx4uujlncz5djvcl54z1t	Running		8m46.347353182s		ip-10-111-2-128		Y		8e5df13ce825
  ├─MySQL		b823q39la019onnnseirk3mz9	Running		9m27.574487904s		ip-10-111-2-211		Y		2a49e1d06fd4
  ├─zenjobs		cl8y6st0uxchveqwl9ahd1ic9	Running		8m4.941223868s		ip-10-111-2-128		Y		5424e645362d
  ├─memcached		cmfdvna9ldduxih8x5d3ltt3l	Running		8m46.356166382s		ip-10-111-2-128		Y		a0b72142f78d
  ├─Zope/0		e4qdux5tuji46i1rcrrew39wk/0	Running		6m29.380297912s		ip-10-111-2-33		Y		7d22aad09692
  ├─Zope/1		e4qdux5tuji46i1rcrrew39wk/1	Running		7m31.120394256s		ip-10-111-2-33		Y		4e28613b26e7
  ├─Zope/2		e4qdux5tuji46i1rcrrew39wk/2	Running		7m40.292449693s		ip-10-111-2-211		Y		4e29fb5c2d81
  ├─Zope/3		e4qdux5tuji46i1rcrrew39wk/3	Running		7m44.787472581s		ip-10-111-2-128		Y		fbf4b568b711
  ├─Zope/4		e4qdux5tuji46i1rcrrew39wk/4	Running		7m30.999586797s		ip-10-111-2-33		Y		91d28116334e
  ├─Zope/5		e4qdux5tuji46i1rcrrew39wk/5	Running		7m51.747193793s		ip-10-111-2-211		Y		77992d7258c8
  └─zeneventd		gtvfl3pfemlxbdfranwft9du	Running		9m33.797065929s		ip-10-111-2-211		Y		6b124eee2fdd
root@ip-10-111-2-211:/var/log/upstart# 
```