GHSA-fm7p-mprw-wjm9

### Summary

`Oj.load` in `:object` mode reads uninitialized stack memory (and, for long keys, reads out of bounds) when parsing a JSON object whose key is 254 bytes or longer. The interned bytes can surface to the caller, disclosing process stack memory.

### Details

In `ext/oj/intern.c`, `form_attr()` handles the long-key path by allocating a heap buffer `b`, populating it with the attribute name, and then freeing it — but it passed the **uninitialized stack buffer `buf`** (not `b`) to `rb_intern3()`:

```c static VALUE form_attr(const char *str, size_t len) { char buf[256]; if (sizeof(buf) - 2 <= len) { // long-key path (len >= 254) char *b = OJ_R_ALLOC_N(char, len + 2); // ... b is filled correctly ... id = rb_intern3(buf, len + 1, oj_utf8_encoding); // BUG: reads `buf` OJ_R_FREE(b); return id; } // ... } ```

`rb_intern3` therefore reads `len + 1` bytes of uninitialized stack memory. When the key length is >= 256, it also reads out of bounds past the 256-byte `buf` (CWE-125). The resulting bytes are interned and can reach the caller via the produced Symbol or via the `EncodingError` message raised on invalid UTF-8, leaking process stack contents.

This is the same defect previously fixed in `ext/oj/usual.c`; `intern.c` held a duplicated copy of `form_attr` that was missed.

### Proof of Concept

```ruby require 'oj' key = "A" * 300 json = %Q[{"^o":"Object","#{key}":1}] Oj.load(json, mode: :object) ```

On affected versions this raises an `EncodingError` whose message contains ~1500 bytes of uninitialized stack memory (not the supplied "A"s). The leaked byte count varies between runs with the identical payload (e.g. 1491 vs 1516 bytes), confirming the content is uninitialized memory rather than fixed data.

### Impact

Information disclosure of process stack memory to a caller that parses untrusted JSON with `Oj.load(..., mode: :object)`. For keys >= 256 bytes it is also an out-of-bounds read (CWE-125).

Severity is bounded by several preconditions: it requires `:object` mode (which is already discouraged for untrusted input), the leaked bytes are uncontrolled (the attacker cannot choose what is disclosed), and the data only reaches an attacker if the application surfaces the resulting Symbol or `EncodingError` back to them. Scored CVSS 5.3 (Medium) on that basis.

### Patches

Fixed in **3.17.3**: `form_attr()` now passes `b` to `rb_intern3` (a one-character change mirroring the earlier `usual.c` fix). Verified on the fixed build: the same payload returns cleanly with no leak across repeated runs.

### Credit

Reported by Zac Wang (@7a6163).