The Reed-Solomon decoder fails silently

[fblomqvi]

As the new tests (53accab) for the RS encoder/decoder show, the decoder sometimes fails silently.

A silent failure means that the decoder announces success but does not return a valid codeword. This is incorrect behaviour. This issue is only encountered when exceeding the error correction capacity, i.e., 2 * #errors + #erasures < minimum_distance. The decoder cannot guarantee correct decoding beyond the error correction capacity. However, the decoder should announce failure whenever it cannot decode the received word to a valid codeword.