[ot] Short-circuit base instance advance variation

src/hb-ot-hmtx-table.hh

diff --git a/src/hb-ot-hmtx-table.hh b/src/hb-ot-hmtx-table.hh
index e710aee..29770f7 100644
--- a/src/hb-ot-hmtx-table.hh
+++ b/src/hb-ot-hmtx-table.hh
@@ -144,7 +144,7 @@
       }
 
       return table->longMetric[MIN (glyph, (uint32_t) num_advances - 1)].advance
-	   + var_table->get_advance_var (glyph, font->coords, font->num_coords); // TODO Optimize?!
+	   + (font->num_coords ? var_table->get_advance_var (glyph, font->coords, font->num_coords) : 0); // TODO Optimize?!
     }
 
     public: