match type of var_entries.used_slots
match type of var_entries.used_slots

`used_slots` and iteration variable `i` did not use matching types causing crashes

this fixes this and changes the types to `zend_long` to match the same struct used in var_unserializer.c in PHP.

rybakit/msgpack-php,msgpack/msgpack-php,msgpack/msgpack-php,msgpack/msgpack-php,rybakit/msgpack-php,rybakit/msgpack-php,rybakit/msgpack-php