fix: don't fail if definition is undefined

Running `uglifyjs --verbose --compress --mangle --screw-ie8 class.js`
with
`class.js`:
```
class Foo {
  bar() {
  }
}
```
Fails with:
```
undefined:4041
  return this.definition().unmangleable(options);
TypeError: Cannot read property 'unmangleable' of undefined
...
```

lib/scope.js

@@ -350,7 +350,8 @@ AST_Scope.DEFMETHOD("references", function(sym){
 });
 
 AST_Symbol.DEFMETHOD("unmangleable", function(options){
-    return this.definition().unmangleable(options);
+    var def = this.definition();
+    return def && def.unmangleable(options);
 });
 
 // property accessors are not mangleable