[subset] Fix null pointer dereference in hvar/vvar subset (#2085)

Rest of the code assumes there is at least one subtable, lets return here if not.

* https://crbug.com/oss-fuzz/19827
* https://crbug.com/oss-fuzz/19847

src/hb-ot-var-hvar-table.hh

@@ -265,6 +265,8 @@ struct hvarvvar_subset_plan_t
     for (unsigned int i = 0; i < inner_maps.length; i++)
       inner_maps[i].init ();
 
+    if (unlikely (!inner_sets.length || !inner_maps.length)) return;
+
     bool retain_adv_map = false;
     index_map_plans[0].init (*index_maps[0], outer_map, inner_sets, plan);
     if (index_maps[0] == &Null(DeltaSetIndexMap))