enforce bounded parsing for NUL-terminated fields in authentication p…

[jmestwa-coder]

Summary

Improve robustness of authentication packet parsing in sql_acl by validating NUL-terminated fields within packet bounds.

Changes

• Replace unbounded string operations (strlen, strend, safe_strlen) with a bounded helper.
• Ensure user, password, database, and plugin fields are parsed within packet limits.
• Handle optional plugin field correctly when absent (next_field == packet_end).

Notes

• No behavior change for valid inputs.
• Malformed packets are now safely rejected instead of causing undefined behaviour.
• Changes are minimal and localized.

[CLAassistant]

All committers have signed the CLA.