Core/LFR: Fix a theoretical error with item_level
Despite the check never happening (player->GetAverageItemLevel() < 0 is impossible), it is more efficient.
Also improves codestyle consistency. Original fix by @emsy.
Merge pull request #11616 from Ascathor/master

Core/LFR: Fix a theoretical error with item_level
Merge branch 'master' of https://github.com/TrinityCore/TrinityCore into ghbb
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