discovery: Handle edge case in scheme-files when looking at symlinks.

discovery: Handle edge case in scheme-files when looking at symlinks.

Previously, this code would cause crashes in Guix (running guix package -s for
example) which could be experienced when Emacs creates temporary files in the
gnu/packages/patches directory when a patch file has been edited, but not
saved.

* guix/discovery.scm (scheme-files): Add else clause to cond used when
handling symlinks.