gccgo: treats 0 as channel

[dvyukov]

gccgo compiles the following incorrect program:

package a
func s() {
    switch <-0 {}
}

gofrontend is on commit 81810917af7ba19e1f9f8efc8b1989f7d6419d30

[dvyukov]

@ianlancetaylor

[gopherbot]

CL https://golang.org/cl/13987 mentions this issue.