Fix T83119: Crash with topology mirror affecting a hidden vertex

Caused by rBba97da21acf2.

For non-topology mirror hidden verts were never respected/included in
EDBM_verts_mirror_cache_begin (they were excluded from the kdtree).

Prior to said commit, hidden mirrored verts that were still in the map
would have been excluded in a separate loop over vertices in
'editmesh_mirror_data_calc()' by checking BM_ELEM_HIDDEN. Due to the new
nature of this function this check was now moved to
EDBM_verts_mirror_cache_begin.

Maniphest Tasks: T83119

Differential Revision: https://developer.blender.org/D9673

source/blender/editors/mesh/editmesh_utils.c

@@ -1139,6 +1139,9 @@ void EDBM_verts_mirror_cache_begin_ex(BMEditMesh *em,
 
     if (use_topology) {
       v_mirr = cache_mirr_intptr_as_bmvert(mesh_topo_store.index_lookup, i);
+      if (respecthide && BM_elem_flag_test(v_mirr, BM_ELEM_HIDDEN)) {
+        v_mirr = NULL;
+      }
     }
     else {
       int i_mirr;