Correctly initialize ptem

src/hb-font.cc

diff --git a/src/hb-font.cc b/src/hb-font.cc
index 8884207..a43656d 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -1124,6 +1124,7 @@
   font->parent = hb_font_get_empty ();
   font->face = hb_face_reference (face);
   font->klass = hb_font_funcs_get_empty ();
+  font->ptem = -1;
 
   font->x_scale = font->y_scale = hb_face_get_upem (face);