SimpleXMLElement adding children incompatibility

### HHVM Version

HipHop VM 3.12.1 (rel)
Compiler: tags/HHVM-3.12.1-0-gf516f1bb9046218f89885a220354c19dda6d8f4d
Repo schema: f2e5f39b2ad4a08bcbd90b5d8bcb580f40fba6c8
### Standalone code, or other way to reproduce the problem

``` php
');
$xml->Child->Sub->OtherSub = "Sub's data";
echo $xml->Child->Sub->OtherSub; // Sub's data
```
### Expected result

Sub's data
### Actual result

Warning: Creating default object from empty value in /var/www/test.php on line 3
Notice: Cannot access property on non-object in /var/www/test.php on line 4