Fix "undefined offset" warning notice on ban list page for IP address bans

[geominorai]

Ban list page does not properly check for a valid SteamID2 before converting to a SteamID3, which happens for the IP address ban type that does not require a Steam ID (defaults to 0 in the _bans table). This produces warnings due to attempting to manually split the string into parts.

Description

Switch to using the SteamID library, which has a built-in validity check.

Motivation and Context

This error is hidden from public view with display_errors = Off in php.ini but should still be corrected.

How Has This Been Tested?

Tested on Linux with Apache 2.4.29 / PHP 7.4.21
Page now loads without the error.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.