Fix the bugs found by Coverity Scan

[rillig]

This project has been submitted to Coverity Scan, a static analyzer. It has found many bugs, most of which have already been fixed. The remaining ones should be fixed as well.

https://scan.coverity.com/projects/4743

If you need access to that site, contact @rillig.

[LBiv]

I'm currently working on a separate branch in which I'm refactoring the interface an naming conventions. I will merge all your recent commits into it before merging and then I will attack the reminder of these bugs.