Previously in
```
edge_y = edge_step(current_vertex_y, edge_y, ¤t_vertex_y);
while (!BM_elem_flag_test(current_vertex_y, BM_ELEM_TAG))
```
With hidden faces this code enters in an infinite loop by returning the same null
edge in edge_step() and the same current_vertex_y causing Blender to crash.
The current fix solves the infinite loop and provides a solution when faces are hidden.
This causes the hidden faces to be merged into one for unsubdivided per original dev's implementation.
At minimum the crash is identified and a solution proposed.
Attached is a video of the fix
Pull Request: https://projects.blender.org/blender/blender/pulls/121086
00e2f55239