sysv-generator: fix wrong "Overwriting existing symlink" warnings

Fix result testing of is_symlink() to ignore negative results, which happen if
the file name does not exist at all. In this case we do not want a warning and
unlink the non-existing link.

https://bugs.debian.org/778700
(cherry picked from commit 4e558983)

Apparently this was missed for some reason and did not end up in
stable previously.