handler.validateReturns() panics on function with no return value

[mlctrez]

The validateReturns method panics with runtime: index out of range for a handler function with no return value. Tested with go v1.9.2

Adding this test case in lambda/handler_test.go:

{
	name:     "no return value should not result in error",
	expected: nil,
	handler: func() {
	},
},

results in

panic: runtime error: index out of range [recovered]
	panic: runtime error: index out of range

goroutine 18 [running]:
testing.tRunner.func1(0xc420152b40)
	/usr/local/go/src/testing/testing.go:711 +0x2d2
panic(0x138f900, 0x15f9800)
	/usr/local/go/src/runtime/panic.go:491 +0x283
reflect.(*rtype).Out(0x1369c00, 0x0, 0x1393940, 0xc420016a88)
	/usr/local/go/src/reflect/type.go:1043 +0xc7
github.com/aws/aws-lambda-go/lambda.validateReturns(0x15d3040, 0x1369c00, 0x1369c00, 0x0)
	/Users/mattman/go/src/github.com/aws/aws-lambda-go/lambda/handler.go:64 +0x1c6

Line 63 in handler.go needs to check for at least one return value before performing the type check.

	} else if handler.NumOut() > 0 {
		if !handler.Out(0).Implements(errorType) {

[bmoffatt]

Feel free to create a PR to fix this up. Otherwise I'll make the change later this afternoon.

[dlsniper]

I've bumped into the same thing and I've sent #9

[mlctrez]

Thanks @dlsniper !