-c with an existing but empty file doesn't work
In this case wget2 produces a unique file (e.g. filename.1). This is not backward compatible with wget1.x.
src/wget.c we have get_file_size(). It should return -1 on error and all calls have to check for the return value resp. the checks have to be modified from
> 0 to
In this process we should also review/modify get_file_mtime(), which could also return an error.