ringbuffer: dif is never less than zero

[YutaMiyake]

Since dif is of type uint64, it can never be less than 0. Instead, when attempting to put an item into a full queue or get an item from an empty queue, dif overflows in the mod calculation manner (e.g., -1 mod 2^64 becomes 18446744073709551615). For clean code, I think it might be better to remove the unreachable code.

[aviary-wf]

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