gofix: avoid panic on body-less functions in netudpgroup.

R=rsc, r
CC=golang-dev
https://golang.org/cl/5347041

src/cmd/gofix/netudpgroup.go

@@ -30,7 +30,7 @@ func netudpgroup(f *ast.File) bool {
 	fixed := false
 	for _, d := range f.Decls {
 		fd, ok := d.(*ast.FuncDecl)
-		if !ok {
+		if !ok || fd.Body == nil {
 			continue
 		}
 		walk(fd.Body, func(n interface{}) {

src/cmd/gofix/netudpgroup_test.go

@@ -28,6 +28,26 @@ func f() {
 	err := x.JoinGroup(nil, gaddr)
 	err = y.LeaveGroup(nil, gaddr)
 }
+`,
+	},
+	// Innocent function with no body.
+	{
+		Name: "netudpgroup.1",
+		In: `package main
+
+import "net"
+
+func f()
+
+var _ net.IP
+`,
+		Out: `package main
+
+import "net"
+
+func f()
+
+var _ net.IP
 `,
 	},
 }