Fix bounds check in test code

[Emik03]

Fix bounds check in test code

A single bounds check from a class used in test code (DiagonsticPoolBlock.cs) has been corrected.

Description

I was implementing MemoryManager<T> in my own project but needed to find an example implementation of the Pin method to ensure I did everything correctly. Looking through grep.app, I found the file this pull request is about and took note of what it did, but then looking closer I saw a bounds check that looked wrong.

Sure enough, this class does allow you to pin the element that comes after the last. (i.e. block.Pin(memoryOwner.Memory.Length)). I'm aware that this code is only used for testing, so as far as bugs are concerned this is very low priority, but it's better to fix it before it causes problems given that the type is in a shared project, implying that this type may be reused and could cause a future problem.

I hope you don't mind me sending a PR without a corresponding issue, as I believe opening an issue for something this small would only slow both me and the maintainers down. The bounds check that I replaced it with is based on what I've seen the most from the runtime repository to maintain some consistency.

[dotnet-policy-service]

Thanks for your PR, @@Emik03. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[dotnet-policy-service]

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