The original looks safe, `is_header` is set at https://github.com/geany/geany-plugins/blob/6e697f2582b03db6749b91da2f247c6e... except when `known_type` is set `FALSE`. Then all uses of `is_header` are guarded by `known_type` being true so its safe.
Probably olde cppcheck simply objected to all uninitialised variables, but new cppcheck is smarter and looks at actual usage.