CBigNum: Convert negative int64 values in a more well-defined way
CBigNum: Convert negative int64 values in a more well-defined way

Since the minimum signed integer cannot be represented as positive so long as its type is signed, and it's not well-defined what happens if you make it unsigned before negating it, we instead increment the negative integer by 1, convert it, then increment the (now positive) unsigned integer by 1 to compensate

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