[buffer] Fix unsafe_to_concat()

Ouch!
diff --git a/src/hb-buffer.hh b/src/hb-buffer.hh
index b0e68e6..f04ad58 100644
--- a/src/hb-buffer.hh
+++ b/src/hb-buffer.hh
@@ -473,7 +473,7 @@
       return;
     _set_glyph_flags (HB_GLYPH_FLAG_UNSAFE_TO_CONCAT,
 		      start, end,
-		      true);
+		      false);
   }
   void unsafe_to_break_from_outbuffer (unsigned int start = 0, unsigned int end = -1)
   {