[ot-font] Only use vmtx side-bearing if table exists

Part of fixing https://github.com/harfbuzz/harfbuzz/issues/537

src/hb-ft.cc

@@ -382,6 +382,7 @@ hb_ft_get_glyph_v_advance (hb_font_t *font,
 
   /* Note: FreeType's vertical metrics grows downward while other FreeType coordinates
    * have a Y growing upward.  Hence the extra negation. */
+
   return (-v + (1<<9)) >> 10;
 }
 #endif

src/hb-ot-font.cc

@@ -161,7 +161,8 @@ hb_ot_get_glyph_v_origin (hb_font_t *font,
   }
 
   hb_glyph_extents_t extents = {0};
-  if (ot_face->glyf->get_extents (font, glyph, &extents))
+  if (ot_face->vmtx->has_data () &&
+      ot_face->glyf->get_extents (font, glyph, &extents))
   {
     const OT::vmtx_accelerator_t &vmtx = *ot_face->vmtx;
     hb_position_t tsb = vmtx.get_side_bearing (font, glyph);

src/hb-ot-hmtx-table.hh

@@ -220,6 +220,8 @@ struct hmtxvmtx
       var_table.destroy ();
     }
 
+    bool has_data () const { return (bool) num_bearings; }
+
     int get_side_bearing (hb_codepoint_t glyph) const
     {
       if (glyph < num_long_metrics)