Core: Fix possible iterator invalidation in event handler

[zach2good]

I affirm:

• [x] I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• [x] I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• [x] I have read and understood the Contributing Guide and the Code of Conduct.
• [x] I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Tested with this on mob init, all expected values printed correctly:

    print("=== GOBLIN")
    print("hasListener ATTACK:", mob:hasListener('ATTACK')) -- expect false
    mob:addListener('ATTACK', 'COLLECTOR_ATTACK', function() end)
    print("hasListener ATTACK:", mob:hasListener('ATTACK')) -- expect true
    mob:removeListener('COLLECTOR_ATTACK')
    print("hasListener ATTACK:", mob:hasListener('ATTACK')) -- expect false

[zach2good]

Tested and now out of draft 👍