(b) That's right. In the example, unknown to A, the wine was destroyed before the sale. Since it is impossible to validly buy and sell something that doesn't exist, the sale of goods legislation makes the agreement void (of no legal effect). There is thus no contractual obligation for B to pay A.
Different rules apply when goods are damaged or destroyed after a sale of the goods. Generally, when identified and deliverable goods are bought and sold, ownership and risk of loss or harm both pass to the buyer as soon as the contract is made (unless otherwise agreed). Accordingly, If the wine had still been in existence when A accepted B's offer, B would have immediately became the owner and carried the risk of any subsequent accidental damage or loss.
When, for some reason, ownership and risk do not pass immediately to the buyer when the sale of the goods is made, and the goods are thereafter destroyed, the sale of goods legislation says the contract is avoided. This leaves the risk on the seller who was the owner when the goods were destroyed.