Fix cycles crash when voxel attributes changed

This could happen e.g. when changing smoke type from flow to domain or
connecting a volume shader with to a domain without an actual flow type
around.

Fixes T58569, T68359

Reviewers: brecht

Maniphest Tasks: T58569, T68359

Differential Revision: https://developer.blender.org/D5478

intern/cycles/render/mesh.cpp

@@ -1091,6 +1091,17 @@ bool Mesh::has_true_displacement() const
   return false;
 }
 
+bool Mesh::has_voxel_attributes() const
+{
+  foreach (const Attribute &attr, attributes.attributes) {
+    if (attr.element == ATTR_ELEMENT_VOXEL) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 float Mesh::motion_time(int step) const
 {
   return (motion_steps > 1) ? 2.0f * step / (motion_steps - 1) - 1.0f : 0.0f;
@@ -2020,15 +2031,7 @@ void MeshManager::device_update_preprocess(Device *device, Scene *scene, Progres
 
     if (need_update && mesh->has_volume) {
       /* Create volume meshes if there is voxel data. */
-      bool has_voxel_attributes = false;
-
-      foreach (Attribute &attr, mesh->attributes.attributes) {
-        if (attr.element == ATTR_ELEMENT_VOXEL) {
-          has_voxel_attributes = true;
-        }
-      }
-
-      if (has_voxel_attributes) {
+      if (mesh->has_voxel_attributes()) {
         if (!volume_images_updated) {
           progress.set_status("Updating Meshes Volume Bounds");
           device_update_volume_images(device, scene, progress);