Add asserts to suppress C28020

[jbzdarkid]

C28020 is a warning which guards against array out-of-bounds issues.
For some reason, PreFast is flagging this check (and the subsequent check) as a potential security risk due to out-of-bounds. Adding an additional assert (even if it's bananas) silences the warning.

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Please don't

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[coveralls]

Coverage remained the same at 100.0% when pulling cde29b3 on jbzdarkid:develop into eaac918 on nlohmann:develop.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!

[ScottHutchinson]

I'm still getting this warning in VS 2019 version 16.8.5. I'm using #pragma warning(disable: 28020) to suppress it for now.