Fix #137984: Sculpt mode crash with certain mesh

This was caused by the material indices not referring to an existing
material slot. Clamping the value to the maximum material index for
the given object fixes the issue.

Pull Request: https://projects.blender.org/blender/blender/pulls/138544

source/blender/draw/intern/draw_sculpt.cc

@@ -16,6 +16,7 @@
 
 #include "BKE_attribute.hh"
 #include "BKE_customdata.hh"
+#include "BKE_material.hh"
 #include "BKE_object.hh"
 #include "BKE_paint.hh"
 
@@ -114,11 +115,14 @@ static Vector<SculptBatch> sculpt_batches_get_ex(const Object *ob,
 
   const Span<int> material_indices = draw_data.ensure_material_indices(*ob);
 
+  const int max_material = BKE_object_material_count_eval(ob);
   Vector<SculptBatch> result_batches(visible_nodes.size());
   visible_nodes.foreach_index([&](const int i, const int pos) {
     result_batches[pos] = {};
     result_batches[pos].batch = batches[i];
-    result_batches[pos].material_slot = material_indices.is_empty() ? 0 : material_indices[i];
+    result_batches[pos].material_slot = material_indices.is_empty() ?
+                                            0 :
+                                            std::min(material_indices[i], max_material);
     result_batches[pos].debug_index = pos;
   });