Fix T76514: Invalid geometry in Alembic crashes Blender

Even though {T76514} is caused by invalid geometry, and thus technically
constitutes a bug in the software that created the Alembic file, I would
like Blender not to crash on importing such a file.

The error in the Alembic file consists of invalid mesh loops, where
consecutive loops refer to the same vertex. The `BKE_mesh_validate()`
can actually correct these errors, so this commit focuses on two things:

- Letting Blender survive the situation until the mesh is loaded, and
- Detecting the error so that `BKE_mesh_validate()` can be called only
  when necessary. This ensures there is only a minimal impact on
  performance when loading actually valid data.

Differential Revision: https://developer.blender.org/D7703

Reviewed By: JacquesLucke

source/blender/blenkernel/intern/mesh_validate.c

@@ -1593,8 +1593,15 @@ void BKE_mesh_calc_edges(Mesh *mesh, bool update, const bool select)
       MLoop *l_prev = (l + (mp->totloop - 1));
       int j;
       for (j = 0; j < mp->totloop; j++, l++) {
-        /* lookup hashed edge index */
-        med_index = POINTER_AS_INT(BLI_edgehash_lookup(eh, l_prev->v, l->v));
+        /* Lookup hashed edge index, if it's valid. */
+        if (l_prev->v != l->v) {
+          med_index = POINTER_AS_INT(BLI_edgehash_lookup(eh, l_prev->v, l->v));
+        }
+        else {
+          /* This is an invalid edge; normally this does not happen in Blender, but it can be part
+           * of an imported mesh with invalid geometry. See T76514. */
+          med_index = 0;
+        }
         l_prev->e = med_index;
         l_prev = l;
       }