Beat panics if log directory cannot be created

[ruflin]

Version: master
Steps to reproduce:

• Put log directory in logging.files.path: where directory cannot be created by filebeat (no permissions)
• Start beat

Panic output

2016/09/08 11:09:48.485527 beat.go:285: CRIT Exiting: error initializing logging: mkdir /var/log/filebeat: permission denied
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xa4108]

goroutine 1 [running]:
panic(0x4cc680, 0xc4200120b0)
        /usr/local/Cellar/go/1.7/libexec/src/runtime/panic.go:500 +0x1a1
github.com/elastic/beats/libbeat/logp.(*FileRotator).Rotate(0xc42016e4c0, 0xc420164930, 0xc4201648c0)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/logp/file_rotator.go:118 +0x48
github.com/elastic/beats/libbeat/logp.(*FileRotator).WriteLine(0xc42016e4c0, 0xc420164930, 0x6e, 0x70, 0x6e, 0x70)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/logp/file_rotator.go:64 +0x48
github.com/elastic/beats/libbeat/logp.send(0x4, 0x2, 0x56080a, 0x5, 0x566499, 0xb, 0xc42004bea8, 0x1, 0x1)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/logp/log.go:64 +0x1c0
github.com/elastic/beats/libbeat/logp.msg(0x2, 0x56080a, 0x5, 0x566499, 0xb, 0xc42004bea8, 0x1, 0x1)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/logp/log.go:84 +0x9d
github.com/elastic/beats/libbeat/logp.Critical(0x566499, 0xb, 0xc42004bea8, 0x1, 0x1)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/logp/log.go:101 +0x71
github.com/elastic/beats/libbeat/beat.handleError(0x7c5080, 0xc420174370, 0x7c5080, 0xc420174370)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/beat/beat.go:285 +0xb3
github.com/elastic/beats/libbeat/beat.Run(0x5643b0, 0x8, 0x0, 0x0, 0x5a97a8, 0xc42004bf90, 0xc4200001a0)
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/libbeat/beat/beat.go:133 +0x7d
main.main()
        /Users/ruflin/Dev/gopath/src/github.com/elastic/beats/filebeat/main.go:22 +0x57