`matrix_stats()` breaks with multiple threads in some cases, when single threaded still works

```
Error in dimnames(x) <- dn:
! length of 'dimnames' [2] not equal to array extent
```

Using a matrix from get_demo_mat(), noticing that matrix_stats() for variance works with 1 thread, but not 4.  Not entirely sure the conditions that cause a breakage, will update this as I find out more.