* fixed possible segfault

Hello, 

I have caught following segmentation fault on one of my servers:
```
#0  0x00007f11de61789e in apc_iterator_search_match (iterator=0x39b49d0, slot=0x7f11bf5a7548) at /usr/build/php/apcu-4.0.6/apc_iterator.c:183
        key = <value optimized out>
        key_len = <value optimized out>
        rval = 1
#1  0x00007f11de61849e in apc_iterator_fetch_active (iterator=0x39b49d0) at /usr/build/php/apcu-4.0.6/apc_iterator.c:241
        count = 56
        slot = <value optimized out>
        item = <value optimized out>
        t = 1417933659
#2  0x000000000074fc32 in zend_call_function (fci=0x7fff0e4b1640, fci_cache=<value optimized out>) at /usr/build/php/php-5.5.11/Zend/zend_execute_API.c:957
#3  0x0000000000775ba7 in zend_call_method (object_pp=0x7fff0e4b1768, obj_ce=<value optimized out>, fn_proxy=0x319c3f0, function_name=0x87caa0 "rewind", 
    function_name_len=<value optimized out>, retval_ptr_ptr=0x0, param_count=0, arg1=0x0, arg2=0x0) at /usr/build/php/php-5.5.11/Zend/zend_interfaces.c:97
```

when `(*slot)->next == NULL` we create reference to null pointer by following code `slot = &(*slot)->next;`
So I have changed type of `slot` to `apc_cache_slot_t *`

Could you merge this changes if it seem reasonable to you.

Kind regards,
Dmitry Saprykin