Reduce calls to malloc in unordered_multiset

[bkthomps]

Using this as a test:

    unordered_multiset me = unordered_multiset_init(sizeof(int), hash_int, compare_int);
    int count = 0;
    int i;
    for (i = 0; i < 1000000; i++) {
        unordered_multiset_put(me, &i);
    }
    for (i = 0; i < 1000000; i++) {
        count += unordered_multiset_contains(me, &i);
    }
    printf("%d\n", count);
    unordered_multiset_destroy(me);

The new version ran in 0.31s and the old version in 0.51s, which improves efficiency by 40%.