Added sequence point in pthreads_store_chunk

Following code snippet was causing a seg fault on my build (Apple LLVM version 5.1 (clang-503.0.40))

```
$obj = new Threaded();

for($n = 0; $n < 100; ++$n){
    $obj[] = $n;
}

var_dump($obj->chunk(10));
```

Added sequence point to prevent ambiguous param evaluation order when calling zend_hash_get_current_key_ex inline with zend_hash_del_key_or_index.