Center unknown marks horizontally

src/hb-ot-shape-fallback.cc

diff --git a/src/hb-ot-shape-fallback.cc b/src/hb-ot-shape-fallback.cc
index 56a3e7a..f1ea61a 100644
--- a/src/hb-ot-shape-fallback.cc
+++ b/src/hb-ot-shape-fallback.cc
@@ -171,6 +171,7 @@
       }
       /* Fall through */
 
+    default:
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_BELOW:
     case HB_UNICODE_COMBINING_CLASS_ATTACHED_ABOVE:
     case HB_UNICODE_COMBINING_CLASS_BELOW: