Fixes <http://bugs.gnu.org/26949>.
Reported by Pjotr Prins <pjotr.public12@thebird.nl>.
* guix/build/union.scm (file-is-directory?): Return #f when FILE does
not exist or is a dangling symlink.
(file=?): Pass #f as a second argument to 'stat'; return #f when both
ST1 or ST2 is #f.
* tests/profiles.scm (test-equalm): New macro.
("union vs. dangling symlink"): New test.