If you've already written part of the review, then don't delete it. You've worked on it, and your effort should not be wasted.
If you haven't written anything yet, then:
If it looks like a simple typo, then:
- If you're 100% sure of the fix, then just fix it, for everyone's sake
- If you're not 100% sure of the fix, you can let others fix it
If the code looks legitimate, you can go ahead and review
If the bug is blatantly obvious, then the question doesn't look legitimate, and it's best to vote to close, and probably downvote too