interp: avoid panic in output value generation for recursive types

Fixes #1134.

_test/issue-1134.go

@@ -0,0 +1,20 @@
+package main
+
+type I interface {
+	Hello()
+}
+
+type T struct {
+	Name  string
+	Child []*T
+}
+
+func (t *T) Hello() { println("Hello", t.Name) }
+
+func main() {
+	var i I = new(T)
+	i.Hello()
+}
+
+// Output:
+// Hello

interp/value.go

@@ -350,7 +350,7 @@ func genValueOutput(n *node, t reflect.Type) func(*frame) reflect.Value {
 		}
 		fallthrough
 	case n.anc.kind == returnStmt && n.anc.val.(*node).typ.ret[0].cat == interfaceT:
-		if len(n.anc.val.(*node).typ.ret[0].field) == 0 {
+		if nod, ok := n.anc.val.(*node); !ok || len(nod.typ.ret[0].field) == 0 {
 			// empty interface, do not wrap
 			return value
 		}