Oj: Use-After-Free in Oj::Parser SAJ Long Key Callback

### Summary `Oj::Parser` in SAJ mode does not protect cached object keys (≥ 35 bytes) from garbage collection. A Ruby callback that triggers GC inside `hash_end` can cause the key string to be reclaimed while the C parser still holds a pointer to it. The subsequent access to the freed string VALUE results in a segfault, confirmed by an RIP pointing to address `0x4242` (a canary-style pattern suggesting control over the freed memory's content). ### Version - **Software**: oj gem - **Affected**: all versions with `ext/oj/saj2.c` / `ext/oj/parser.c` - **Latest tested**: 3.17.1 (confirmed present) ### Details Short keys (≤ 34 bytes) are stored inline on the C stack and are safe. Long keys (≥ 35 bytes) are stored as heap-allocated Ruby String objects passed to `rb_funcall` as the `key` argument. Between the key being resolved and the callback completing, a GC triggered inside the callback (e.g. `GC.start`) can collect the key String, leaving a dangling VALUE. Crash output: ``` long_key_trigger [BUG] Segmentation fault at 0x0000000000004242 close_object+0x260 /ext/oj/usual.c:405 (calls rb_funcall with freed key) parse+0x11ff /ext/oj/parser.c:693 parser_parse+0x145 /ext/oj/parser.c:1408 RIP: 0x7fd1b46d68b7 RDI: 0x0000000000004242 (freed key VALUE) R12: 0x0000000000004242 ``` The freed VALUE `0x4242` shows the attacker-controlled content of the key string was loaded as a pointer — a classic use-after-free indicator. ### Reproduce ```ruby require 'oj' class H < Oj::Saj def add_value(value, key) GC.start(full_mark: true, immediate_sweep: true) if key == 'x' end def hash_start(key); end def hash_end(key); end end p = Oj::Parser.new(:saj) p.handler = H.new p.parse('{"' + 'A' * 35 + '":{"x":1}}') # long outer key, GC fires on inner key ```

Quoted source text, attributed separately from HOL analysis.