invalid mutation for `var _ = ...` about gofumpt

[mvdan]

Thanks - indeed forgot about an edge case.

[mmcloughlin]

$ cat main.go
package main

func main() {
	var _ = "Hello, World!"
}
$ gofumpt main.go
package main

func main() {
	_ := "Hello, World!"
}
$ gofumports main.go
package main

func main() {
	_ := "Hello, World!"
}
$ go build main.go
$ gofumpt -w main.go
$ go build main.go
# command-line-arguments
./main.go:4:4: no new variables on left side of :=