Fix T71311 Bridge Edge Loops crash when bridging cube edges

This commit will prevent the crash. It does produce a result that may not
be desirable (only briding one face of the cube), but at least it's better
than crashing.

source/blender/bmesh/operators/bmo_subdivide_edgering.c

@@ -828,6 +828,11 @@ static void bm_face_slice(BMesh *bm, BMLoop *l, const int cuts)
   for (i = 0; i < cuts; i++) {
     /* no chance of double */
     BM_face_split(bm, l_new->f, l_new->prev, l_new->next->next, &l_new, NULL, false);
+    if (l_new == NULL) {
+      /* This happens when l_new->prev and l_new->next->next are adjacent. Since
+       * this sets l_new to NULL, we cannot continue this for-loop. */
+      break;
+    }
     if (l_new->f->len < l_new->radial_next->f->len) {
       l_new = l_new->radial_next;
     }