example gives warning

Hi,

I ran your example in `item_sim` and it gave an odd warning. This is from the version 2.1.2

```
item_gen(b_bounds = c(-2, 2), a_bounds = c(.75, 1.25),
thresholds = c(1, 2, 3), n_1pl = c(5, 5, 5), n_2pl = c(0, 0, 5))
# Warning message:
# In rbind(c(0, 0, 0), c(0, 0, 0), c(0, 0, 0), c(0, 0, 0), c(0, 0,  :
#   number of columns of result is not a multiple of vector length (arg 6)
```