🐛 Fix UBL 'G29 Q1' bug

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

 
       case 1:
         LOOP_L_N(x, GRID_MAX_POINTS_X) {                     // Create a diagonal line several Mesh cells thick that is raised
+          const uint8_t x2 = x + (x < (GRID_MAX_POINTS_Y) - 1 ? 1 : -1);
           z_values[x][x] += 9.999f;
-          z_values[x][x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1] += 9.999f; // We want the altered line several mesh points thick
+          z_values[x][x2] += 9.999f; // We want the altered line several mesh points thick
           #if ENABLED(EXTENSIBLE_UI)
             ExtUI::onMeshUpdate(x, x, z_values[x][x]);
-            ExtUI::onMeshUpdate(x, (x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1), z_values[x][x + (x < (GRID_MAX_POINTS_Y) - 1) ? 1 : -1]);
+            ExtUI::onMeshUpdate(x, (x2), z_values[x][x2]);
           #endif
-
         }
         break;