Add back a null check

This was accidentally dropped in the previous commit.

src/hb-ot-cmap-table.hh

@@ -1898,6 +1898,8 @@ struct cmap
     bool get_nominal_glyph (hb_codepoint_t  unicode,
                             hb_codepoint_t *glyph) const
     {
+      if (unlikely (!this->get_glyph_funcZ)) return false;
+
       return this->get_glyph_funcZ (this->get_glyph_data, unicode, glyph);
     }