MyBB: Security Question insufficient validation

### Impact
The registration component does not validate the text-based _Security Question_ CAPTCHA correctly, allowing attackers to bypass the challenge via a specially crafted value.

[CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N](https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N)

### Details
The public _Registration_ workflow ([`member.php?action=do_register`](https://github.com/mybb/mybb/blob/mybb_1839/member.php#L262-L307)) accepts a hidden field `question_id` — expected to match the question session identifier (`mybb_questionsessions.sid`) — and validates the challenge answer without a fail-closed fallback for invalid identifiers. If the value is blank, forged, or expired, the request continues without a question-related error.

### Patches
MyBB 1.8.(...) resolves this issue with the following changes:

- Commit: https://github.com/mybb/mybb/commit/
- `.patch`: https://github.com/mybb/mybb/commit/.patch

### References
- Release Notes: https://mybb.com/versions/1.8.(...)/

### For more information
Go to [mybb.com/security](https://mybb.com/security/) to report possible security concerns or to learn more about security research at MyBB.

### Contact
The security team can be reached at [security@mybb.com](mailto:security@mybb.com).