ItemRepository::index: fix off-by-one error
Several member functions of ItemRepository access elements of m_buckets at indices up to and including m_currentBucket. Presumably m_currentBucket < m_buckets.size() is an invariant. needMonsterExtent + 1 is added to m_currentBucket a few lines below the modified line. So when m_currentBucket + needMonsterExtent + 1 == m_buckets.size(), the size of m_buckets must be increased. The bug fixed in this commit caused frequent assertion failures in ItemRepository when TestItemRepository::testItemRepository() randomly picked a large item size - `if (itemDecision < largeItemsFraction)`.