Quiz end with invalid attempt key causes fatal error

[brianhogg]

Reproduction Steps

I believe this would only happen if someone is maliciously trying to change the attempt ID when ending a quiz.

Expected Behavior

No fatal error

Actual Behavior

Error message shown below if the attempt key for the quiz is invalid.

Error Messages / Logs

• Include any relevant error messages or log files

NOTICE: PHP message: PHP Fatal error:  Uncaught Error: Call to a member function end() on bool in /www/wp-content/plugins/lifterlms/includes/class.llms.ajax.handler.php:953"

This issue has be recreated:

• Locally
• On a staging site
• On a production website
• With only LifterLMS and a default theme

HS-250772