interp: fix type assertion issues

closes #1514

hi!
I had the same problem as #1514 and I wanted to fix it, I found
When asserting *crypto/rsa.PublicKey, using the typ attribute of node to get an nil rtype, resulting in the assertion result being nok

This code contains the same problem
```go
package main

import (
	"log"
	"crypto/rsa"
)

func main() {

    var pKey interface{} = &rsa.PublicKey{}

    if _, ok := pKey.(*rsa.PublicKey); ok {
        log.Println("ok")
    } else {
        log.Println("nok")
    }
}

```

So I submitted this Pull Request, hope it will be merged

_test/assert3.go

@@ -0,0 +1,16 @@
+package main
+
+import "crypto/rsa"
+
+func main() {
+	var pKey interface{} = &rsa.PublicKey{}
+
+	if _, ok := pKey.(*rsa.PublicKey); ok {
+		println("ok")
+	} else {
+		println("nok")
+	}
+}
+
+// Output:
+// ok

interp/run.go

@@ -290,7 +290,7 @@ func typeAssert(n *node, withResult, withOk bool) {
 
 	typ := c1.typ // type to assert or convert to
 	typID := typ.id()
-	rtype := typ.rtype // type to assert
+	rtype := typ.refType(nil) // type to assert
 	next := getExec(n.tnext)
 
 	switch {