Fix T88658: Force Fields of curve shape can crash if curve has only one point

`bvhtree_from_mesh_edges_create_tree` can actually leave the BVHTree
NULL (e.g. if no edges are present).

Now dont allocate `BVHTreeFromMesh` on the `SurfaceModifierData` at all
in case the tree would be NULL anyways.
Places like `get_effector_data` check for `SurfaceModifierData`-
>`BVHTreeFromMesh` and dont try to stuff like getting a closest point on
surface, which would crash as soon as BVHNodes would need to be accessed
(from the NULL BVHTree).

Maniphest Tasks: T88658

Differential Revision: https://developer.blender.org/D11443

1 files changed, 10 insertions, 6 deletions

diff --git a/source/blender/modifiers/intern/MOD_surface.c b/source/blender/modifiers/intern/MOD_surface.c
index d2c011a21d3..bfd4cd81803 100644
--- a/source/blender/modifiers/intern/MOD_surface.c
+++ b/source/blender/modifiers/intern/MOD_surface.c
@@ -184,13 +184,17 @@ static void deformVerts(ModifierData *md,
 
     surmd->cfra = cfra;
 
-    surmd->bvhtree = MEM_callocN(sizeof(BVHTreeFromMesh), "BVHTreeFromMesh");
+    const bool has_poly = surmd->mesh->totpoly > 0;
+    const bool has_edge = surmd->mesh->totedge > 0;
+    if (has_poly || has_edge) {
+      surmd->bvhtree = MEM_callocN(sizeof(BVHTreeFromMesh), "BVHTreeFromMesh");
 
-    if (surmd->mesh->totpoly) {
-      BKE_bvhtree_from_mesh_get(surmd->bvhtree, surmd->mesh, BVHTREE_FROM_LOOPTRI, 2);
-    }
-    else {
-      BKE_bvhtree_from_mesh_get(surmd->bvhtree, surmd->mesh, BVHTREE_FROM_EDGES, 2);
+      if (has_poly) {
+        BKE_bvhtree_from_mesh_get(surmd->bvhtree, surmd->mesh, BVHTREE_FROM_LOOPTRI, 2);
+      }
+      else if (has_edge) {
+        BKE_bvhtree_from_mesh_get(surmd->bvhtree, surmd->mesh, BVHTREE_FROM_EDGES, 2);
+      }
     }
   }
 }