[ BUG ] nil target item cause prometheus exit

[Colstuwjx]

What did you do?
using a file sd to define the target groups.

What did you expect to see?
wrong discovery data should be skipped or warning, not panic.

What did you see instead? Under which circumstances?
panic, and the details is:

INFO[0000] Starting target manager...                    source="targetmanager.go:63"

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x166053a]

goroutine 52 [running]:
github.com/prometheus/prometheus/discovery/file.readFile(0xc42015e2a0, 0x1b, 0x1, 0x1, 0xc420b4a270, 0x3, 0x3)
	/Users/Jacky/Jacky.Wu/MyProjects/Go/src/github.com/prometheus/prometheus/discovery/file/file.go:259 +0x1ea
github.com/prometheus/prometheus/discovery/file.(*Discovery).refresh(0xc420b28080, 0x3ea2080, 0xc420b28100, 0xc42001d620)
	/Users/Jacky/Jacky.Wu/MyProjects/Go/src/github.com/prometheus/prometheus/discovery/file/file.go:196 +0xfe
github.com/prometheus/prometheus/discovery/file.(*Discovery).Run(0xc420b28080, 0x3ea2080, 0xc420b28100, 0xc42001d620)
	/Users/Jacky/Jacky.Wu/MyProjects/Go/src/github.com/prometheus/prometheus/discovery/file/file.go:121 +0x198
created by github.com/prometheus/prometheus/discovery.(*TargetSet).updateProviders
	/Users/Jacky/Jacky.Wu/MyProjects/Go/src/github.com/prometheus/prometheus/discovery/discovery.go:249 +0x283

Environment

• System information:

  Linux 3.16.0-4-amd64 x86_64

• Prometheus version:

 prometheus, version 1.6.0 (branch: master, revision: 10f645382988add09c8e3420d0d7ff2dc4d36a29)
  build user:       root@0816a56aa81c
  build date:       20170414-18:36:18
  go version:       go1.8.1

• Alertmanager version:

• Prometheus configuration file:

...
  - job_name: 'nil-item-array-json-job'
    metrics_path: /metrics
    file_sd_configs:
      - files:
         - "invalid_nil_item_array.json"

# invalid_nil_item_array.json
# !!! the last null line caused panic!

[
    {
        "labels": {
            "env": "prod"
        },
        "targets": [
            "localhost:9090"
        ]
    },
    null
]

[lock]

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