Fix #128958: UV unwrap crash with "Use Subdivision" option

When the modifier is disabled with level 0, the corner_vert array
wasn't properly created in the subdivided mesh. The simplest
solution is to just skip the subdiv processing for the unwrap when
the level is 0 and nothing would happen anyway.

Pull Request: https://projects.blender.org/blender/blender/pulls/129447

source/blender/editors/uvedit/uvedit_unwrap_ops.cc

@@ -222,7 +222,15 @@ static void modifier_unwrap_state(Object *obedit,
    * only if modifier is first or right after mirror. */
   if (subsurf) {
     if (md && md->type == eModifierType_Subsurf) {
-      subsurf = true;
+      const SubsurfModifierData &smd = *reinterpret_cast<const SubsurfModifierData *>(md);
+      if (smd.levels > 0) {
+        /* Skip all calculation for zero subdivision levels, similar to the way the modifier is
+         * disabled in that case. */
+        subsurf = true;
+      }
+      else {
+        subsurf = false;
+      }
     }
     else {
       subsurf = false;