Fix Statsd input panic when processing DataDog events

[CriGoT]

Fixes the folllowing panic that is caused by sending a Datadog event to the StatsD input

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1747ff7]

goroutine 12 [running]:
github.com/influxdata/telegraf/plugins/inputs/statsd.(*Statsd).parseEventMessage(0xc0001b8380, 0xbf434fd5f15bd546, 0xd7be3d60, 0x4554680, 0xc000286380, 0x3a, 0xc00
04b8060, 0x9, 0x1, 0x0)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/statsd/datadog.go:136 +0x10a7
github.com/influxdata/telegraf/plugins/inputs/statsd.(*Statsd).parser(0xc0001b8380, 0x2695778, 0xc0001b8408)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/statsd/statsd.go:502 +0x2fd
github.com/influxdata/telegraf/plugins/inputs/statsd.(*Statsd).Start.func4(0xc0001b8380)
        /go/src/github.com/influxdata/telegraf/plugins/inputs/statsd/statsd.go:401 +0x5a
created by github.com/influxdata/telegraf/plugins/inputs/statsd.(*Statsd).Start
        /go/src/github.com/influxdata/telegraf/plugins/inputs/statsd/statsd.go:399 +0x9a7

The PR adds the initialization of the accumulator used by the parseEventMessage function

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

Thank you