Matrix can have negative dimensions, which should raise ValueError when encountered.

```
>>> A = matrix(2, -1)
>>> A.rows, A.cols
(2, -1)
>>> matrix(-1, 2) * matrix(2, -1)
matrix(
[])
>>> _.rows, _.cols
(-1, -1)
>>> A = matrix(2, 2)
>>> A.cols = -1
>>> A.rows = -1
>>> A.cols, A.rows
(-1, -1)
>>> A
matrix(
[])
>>> det(A)
1
```