ibase_close() does not check proper resource type

One can pass any resource to `ibase_close()` and it will reduce ref count:

```php
$gz = gzopen("/dev/null", "w");
var_dump(ibase_close($gz));
var_dump(gzwrite($gz, "data"));
```

> 
> bool(true)
> 
> Fatal error: Uncaught TypeError: gzwrite(): supplied resource is not a valid stream resource

Related to #22