ftruncate/truncate is not fully supported

According to the man:

If the file previously was smaller than this size, ftruncate() shall increase the size of the file. If the file size is increased, the extended area shall appear as if it were zero-filled. The value of the seek pointer shall not be modified by a call to ftruncate().

 auto size = md->size();
    if(static_cast<unsigned long>(length) > size) {
        LOG(DEBUG, "Length is greater then file size: {} > {}", length, size);
        errno = EINVAL;
        return -1;