WinHelp backend segfaults when it finds a nonexistent cross-

reference. (The broken link will already have been reported in the
mid-end, but this won't inhibit output generation, so the onus is
still on us to at the very least not go *crash*.)

git-svn-id: svn://svn.tartarus.org/sgt/halibut@2507 cda61777-01e9-0310-a592-d414129be87e