Fix position sync on M420 S0

Marlin/src/gcode/bedlevel/M420.cpp

     }
   #endif
 
+  xyz_pos_t oldpos = current_position;
+
   // If disabling leveling do it right away
   // (Don't disable for just M420 or M420 V)
   if (seen_S && !to_enable) set_bed_leveling_enabled(false);
 
-  xyz_pos_t oldpos = current_position;
-
   #if ENABLED(AUTO_BED_LEVELING_UBL)
 
     // L to load a mesh from the EEPROM