Fix broken LEVEL_BED_CORNERS

Fix #8805

Marlin/src/Marlin.cpp

       disable_e_steppers();
     #endif
     #if ENABLED(AUTO_BED_LEVELING_UBL) && ENABLED(ULTIPANEL)  // Only needed with an LCD
-      ubl.lcd_map_control = defer_return_to_status = false;
+      if (ubl.lcd_map_control) ubl.lcd_map_control = defer_return_to_status = false;
     #endif
   }
 

Marlin/src/gcode/control/M17_M18_M84.cpp

     }
 
     #if ENABLED(AUTO_BED_LEVELING_UBL) && ENABLED(ULTIPANEL)  // Only needed with an LCD
-      ubl.lcd_map_control = defer_return_to_status = false;
+      if (ubl.lcd_map_control) ubl.lcd_map_control = defer_return_to_status = false;
     #endif
   }
 }