  This note is a corrigendum to the previous version arXiv:0711.2735v3
published in J. Lie Theory. As it has been recently pointed out to me by
Alexander Premet, Remark 3 of arXiv:0711.2735v3 is incorrect. We verify in this
note thanks to recent results of Premet and Topley (see arXiv:1301.4653) that
Theorem 25 of arXiv:0711.2735v3 remains correct in spite of this error.
