Don't deserialize positions if buffer has no positions

src/hb-buffer-serialize.cc

diff --git a/src/hb-buffer-serialize.cc b/src/hb-buffer-serialize.cc
index 7839cbc..2dc146a 100644
--- a/src/hb-buffer-serialize.cc
+++ b/src/hb-buffer-serialize.cc
@@ -282,6 +282,9 @@
   assert ((!buffer->len && buffer->content_type == HB_BUFFER_CONTENT_TYPE_INVALID) ||
 	  buffer->content_type == HB_BUFFER_CONTENT_TYPE_GLYPHS);
 
+  if (!buffer->have_positions)
+    flags |= HB_BUFFER_SERIALIZE_FLAG_NO_POSITIONS;
+
   if (unlikely (start == end))
     return 0;