cmd/fix: check type assertion in netipv6zone rule.

Fixes #5461.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9947043

src/cmd/fix/netipv6zone.go

@@ -51,7 +51,7 @@ func netipv6zone(f *ast.File) bool {
 						Value: e,
 					}
 				case 1:
-					if e.(*ast.BasicLit).Value == "0" {
+					if elit, ok := e.(*ast.BasicLit); ok && elit.Value == "0" {
 						cl.Elts = append(cl.Elts[:i], cl.Elts[i+1:]...)
 					} else {
 						cl.Elts[i] = &ast.KeyValueExpr{

src/cmd/fix/netipv6zone_test.go

@@ -20,6 +20,8 @@ func f() net.Addr {
 	sub(&net.UDPAddr{ip2, 12345})
 	c := &net.TCPAddr{IP: ip3, Port: 54321}
 	d := &net.TCPAddr{ip4, 0}
+	p := 1234
+	e := &net.TCPAddr{ip4, p}
 	return &net.TCPAddr{ip5}, nil
 }
 `,
@@ -32,6 +34,8 @@ func f() net.Addr {
 	sub(&net.UDPAddr{IP: ip2, Port: 12345})
 	c := &net.TCPAddr{IP: ip3, Port: 54321}
 	d := &net.TCPAddr{IP: ip4}
+	p := 1234
+	e := &net.TCPAddr{IP: ip4, Port: p}
 	return &net.TCPAddr{IP: ip5}, nil
 }
 `,