GHSA-2cw7-v8ff-p88r

### Summary

Disabling `symbol_keys` on a reused `Oj::Parser` instance triggers a heap use-after-free. When `symbol_keys` is toggled from `true` to `false`, `opt_symbol_keys_set` frees the internal key cache (`cache_free`) but does not clear the pointer. The next `parse` call reads from the freed cache via `cache_intern`, producing a use-after-free.

### Version

- **Software**: oj gem - **Affected**: all versions with `ext/oj/usual.c` - **Latest tested**: 3.17.1 (confirmed present)

### Details

`ext/oj/usual.c`, `opt_symbol_keys_set`:

```c // usual.c:1043–1051 if (symbol_keys) { d->key_cache = cache_create(...); // allocate } else { cache_free(d->key_cache); // free — but d->key_cache pointer not NULLed } ```

On the next parse call, `cache_key` → `cache_intern` reads from `d->key_cache` which now points to freed memory.

ASAN report: ``` ==145265==ERROR: AddressSanitizer: heap-use-after-free on address 0x50b00001a318 READ of size 8 at 0x50b00001a318 thread T0 #0 cache_intern /ext/oj/cache.c:328 #1 cache_key /ext/oj/usual.c:161 #2 close_object /ext/oj/usual.c:285 #3 parse /ext/oj/parser.c:693 #4 parser_parse /ext/oj/parser.c:1408 freed by thread T0 here: #0 free #1 cache_free /ext/oj/cache.c:277 #2 opt_symbol_keys_set /ext/oj/usual.c:1051 #3 option /ext/oj/usual.c:1111 #4 parser_missing /ext/oj/parser.c:1362 0x50b00001a318 is 40 bytes inside freed 112-byte region [fd]fd fd fd fd fd fd fd ```

### Reproduce

```ruby require 'oj' p = Oj::Parser.new(:usual, symbol_keys: true) p.symbol_keys = false # frees cache without nulling pointer p.parse('{"attacker":1}') # UAF: reads freed cache ```