FOGSSH::disconnect(): 'Undefined array key "_link"' warning on reuse, TypeError without a live session #1997

Problem

• Services write PHP Warning: Undefined array key "_link" in src/Net/FOGSSH.php on line 133 to servicemaster.log. This box logged 285 of them.
• FOGSSH::disconnect() without a live SSH session throws TypeError: ssh2_disconnect(): Argument #1 ($session) must be of type resource. That happens after a failed connect() (link is false) or after an earlier disconnect() (link is null).

Cause

• disconnect() calls unset($this->_link) and unset($this->_sftp). PHP routes the next read of an unset declared property to __get(). __get() reads $this->data['_link'], which does not exist. FOGSSH is one shared instance, so the next connect() or disconnect() warns.
• disconnect() passes the link to ssh2_disconnect() without checking it. On PHP 8, a non-resource argument is a TypeError, and @ does not suppress it.

Fix

• Set both properties to null. Do not unset() them.
• Call ssh2_disconnect() only when the link is a resource.

Affects working-1.6 and rc-1.6.0, including RC18.