[swfinterp] Add support for void methods

test/swftests/PrivateVoidCall.as

@@ -0,0 +1,22 @@
+// input: []
+// output: 9
+
+package {
+public class PrivateVoidCall {
+    public static function main():int{
+        var f:OtherClass = new OtherClass();
+        f.func();
+        return 9;
+    }
+}
+}
+
+class OtherClass {
+    private function pf():void {
+        ;
+    }
+
+    public function func():void {
+        this.pf();
+    }
+}