go/ast: FuncType.Params may be nil (per AST documentation)

ast.Walk needs to check for it or it will crash.

R=r
CC=golang-dev
https://golang.org/cl/6852062

src/pkg/go/ast/walk.go

@@ -158,7 +158,9 @@ func Walk(v Visitor, node Node) {
 		Walk(v, n.Fields)
 
 	case *FuncType:
-		Walk(v, n.Params)
+		if n.Params != nil {
+			Walk(v, n.Params)
+		}
 		if n.Results != nil {
 			Walk(v, n.Results)
 		}