auth: Fix out-of-bounds check in ssh_userauth_kbdint_getprompt()

Arman Alirequested to merge
arman-ali07/libssh-mirror:fix-kbdint-oob into master

Fix out-of-bounds access in ssh_userauth_kbdint_getprompt().

The function previously checked: if (i > session->kbdint->nprompts)

This allowed i == nprompts, which could lead to out-of-bounds access to prompts[i] and echo[i].

The condition has been corrected to: if (i >= session->kbdint->nprompts)

This prevents invalid memory access.

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated
  • The project pipelines timeout is extended at least to 2 hours.

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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