Whoever lends shares to the short seller lends the voting rights attached to the shares as well, so as long as the short is covered, there's not actually any excess voting available. Moreover, sellers have up to two days to deliver the shares to begin with (the settlement period). If a naked short seller can secure the loan or the securities within that period, it all works out. (If they can't, well, it's fraudulent for other reasons more direct than the voting.)