ckd_calloc_[234]d() out-of-bounds read

The following leads to an out-of-bounds read in ```ckd_free_2d``` (can be observed with valgrind of AddressSanitizer). It happens whenever the first argument to ```ckd_calloc_2d``` is 0. This also applies to the ```3d``` and ```4d``` counterparts.

```c
#include <sphinxbase/ckd_alloc.h>

int main(void)
{
    void* p = ckd_calloc_2d(0, 2, 3);
    ckd_free_2d(p);
    return 0;
}
```