Fix floating point exception

[boretom]

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [ ] All tests are passing
• [ ] New/updated tests are included

If adding a new feature, the PR's description includes:

• [ ] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: Ok, I'm pretty sure I got it this time.

Btw: The floating point will happen with all images, it just depends on the -H value. E.g. catimg -H 80 test-images/mewtwo-front.png also triggers it.

Btw2: I started with a branch where I added simple CMake tests feature/add-simple-tests just in case you're bored. It merged with every branch. Two lines are added to the end of CMakeLists.txt and a new directory test. One can run it by calline ctest after make or make test

[posva]

What test did you add? A visual one?