Fix pooled list bug.
Fix pooled list bug.

Ported from upstream: https://github.com/jtmueller/Collections.Pooled/pull/44

SuperJMN/Avalonia,SuperJMN/Avalonia,grokys/Perspex,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia,grokys/Perspex,SuperJMN/Avalonia,AvaloniaUI/Avalonia,AvaloniaUI/Avalonia,SuperJMN/Avalonia,SuperJMN/Avalonia,AvaloniaUI/Avalonia