(bug 15351) Fix fatal error for invalid section fragments in autocomments

(bug 15351) Fix fatal error for invalid section fragments in autocomments

. . . probably.  Actually I can't reproduce the problem, so I can't be sure if this fixes it.  But it should.  I'm not at all sure if this is a real fix or more of a workaround.  One code path (!$local) normalizes the fragment and rejects it as invalid, while the other code path ($local) sets the fragment unquestioningly and doesn't validate it.  I don't know which behavior is correct.