[unsafe-to-break] Mark unsafe is cluster merging is disabled

We were relying on cluster merges not requiring unsafe flagging because
they get merged.  If cluster level requests no merging, then we flag
unsafe when merge would have happened.

src/hb-buffer.cc

@@ -550,7 +550,10 @@ hb_buffer_t::merge_clusters_impl (unsigned int start,
 				  unsigned int end)
 {
   if (cluster_level == HB_BUFFER_CLUSTER_LEVEL_CHARACTERS)
+  {
+    unsafe_to_break (start, end);
     return;
+  }
 
   unsigned int cluster = info[start].cluster;