Is the prayer of one who, having erred in his last prostration, did not know how many prostrations he had made, then unintentionally felt wind exit him during the prostration of forgetfulness, permissible?

If you prostrate for forgetfulness (sujud al-sahw) before the salām and then invalidate your ablution (حدثت), both your purification (ṭahāra) and your prayer become null and void. You must discontinue the prayer, perform ablution, and restart the prayer. However, if you prostrate for forgetfulness after the salām and then invalidate your ablution, your prayer is valid because the invalidation occurred after its completion. You then only need to perform ablution and the prostration of forgetfulness.