Cleanup two divide-by-zero issues from Coverity

[jwrober]

No related issue.

CID 1435482, 1435595

[lmoureaux]

Pushed a commit using the same technique for aunit and punit. This makes the div by zero truly impossible by checking beforehand.

We could also have done some math and used a/b < c/d <=> a*d < b*c (for positive b and d), but this has different rounding which might be important.

[jwrober]

Thanks for the help on this one.