Backend crash with metrics using a handler with the not_silenced filter

[palourde]

Expected Behavior

The backend shouldn't crash!

Current Behavior

If I have a handler, say slack, that use the not_silenced filter, and I use this handler as --statsd-event-handlers on the agent, sensu-backend will crash:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x180 pc=0x1ac953f]
goroutine 184 [running]:
github.com/sensu/sensu-go/types.(*Event).IsSilenced(...)
    /Users/splourde/golang/src/github.com/sensu/sensu-go/types/event.go:137
github.com/sensu/sensu-go/backend/pipelined.(*Pipelined).filterEvent(0xc4202f3420, 0xc4290809c0, 0xc429396140, 0xc42932acf0)
    /Users/splourde/golang/src/github.com/sensu/sensu-go/backend/pipelined/filter.go:110 +0x4ff
github.com/sensu/sensu-go/backend/pipelined.(*Pipelined).handleEvent(0xc4202f3420, 0xc429396140, 0xc42918af20, 0x0)
    /Users/splourde/golang/src/github.com/sensu/sensu-go/backend/pipelined/handle.go:55 +0x3ce
github.com/sensu/sensu-go/backend/pipelined.(*Pipelined).createPipelines.func1(0xc4202f3420, 0xc429087860)
    /Users/splourde/golang/src/github.com/sensu/sensu-go/backend/pipelined/pipelined.go:129 +0x155
created by github.com/sensu/sensu-go/backend/pipelined.(*Pipelined).createPipelines
    /Users/splourde/golang/src/github.com/sensu/sensu-go/backend/pipelined/pipelined.go:117 +0x70

Possible Solution

The event.IsSilenced() method should ensure a check exists!

Steps to Reproduce (for bugs)

Context

Your Environment

• Sensu version used (sensuctl, sensu-backend, and/or sensu-agent): master at this time
• Installation method (packages, binaries, docker etc.): manually
• Operating System and version (e.g. Ubuntu 14.04): macOS latest