Prevent division by zero and force ghostscript run

The `scale` variable can be zero since it's an int. This code should
probably be refactored to use floats, but since no one touched it in
years, this is a quick workaround to not make it crash and to get a
working thumbnail by forcing a ghostscript run with the `break;`.

BUG: 406463
FIXED-IN: 19.12.1