ref-filter: fix memory leak in `free_array_item()`

ref-filter: fix memory leak in `free_array_item()`

We treat the `value` pointer as a pointer to a struct and free its `s`
field. But `value` is in fact an array of structs. As a result, we only
free the first `s` out of `used_atom_cnt`-many and leak the rest. Make
sure we free all items in `value`.

In the caller, `ref_array_clear()`, this means we need to be careful not
to zero `used_atom_cnt` until after we've called `free_array_item()`. We
could move just a single line, but let's keep related things close
together instead, by first handling `array`, then `used_atom`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>