[RFC] Coverity Issues

This PR fixes several Coverity issues listed in #760 , namely 13796, 62607, 62608, 62609, 62612, 62615. Additionally I found that some of the Coverity defects are false positives, especially in the ressource leak category.

I couldn't find how to mark them as false positives in the Coverity web interface though. I suspect that my role of contributer doesn't have sufficient rights to do so.
A buffer can only have `LONG_MAX` lines (`ml_line_count` is of type `long`). So trying to add more than `LONG_MAX` lines is already an error.