Handle class=0 glyphs in a ClassDef correctly

As reported by cibu.  Fixed in HarfBuzz also.
diff --git a/pyotlss.py b/pyotlss.py
index 8a0d504..5662619 100755
--- a/pyotlss.py
+++ b/pyotlss.py
@@ -64,6 +64,10 @@
 @add_method(fontTools.ttLib.tables.otTables.ClassDef)
 def intersects_class (self, glyphs, klass):
 	"Returns true if any of glyphs has requested class."
+	if klass == 0:
+		if any (g not in self.classDefs.items() for g in glyphs):
+			return True
+		# Fall through
 	return any (g in glyphs for g,v in self.classDefs.items() if v == klass)
 
 @add_method(fontTools.ttLib.tables.otTables.ClassDef)