Problem: zlistx_head always return null

[somdoron]

Solution: Fix the function and return the first item

For comparison, following are the implementations of zlistx_head and zlist_first (actually works) before the change, the difference should only be the cursor change, however the implementation is different:

void *
zlistx_head (zlistx_t *self)
{
    assert (self);
    return self->head? self->head->item: NULL;
}

void *
zlistx_first (zlistx_t *self)
{
    assert (self);
    self->cursor = self->head->next;
    return self->cursor == self->head? NULL: self->cursor->item;
}