Allow Z_SAFE_HOMING_POINT outside bed (#16945)

Marlin/src/inc/SanityCheck.h

  * Make sure Z_SAFE_HOMING point is reachable
  */
 #if ENABLED(Z_SAFE_HOMING)
-  static_assert(WITHIN(Z_SAFE_HOMING_X_POINT, X_MIN_BED, X_MAX_BED), "Z_SAFE_HOMING_X_POINT can't be reached by the nozzle.");
-  static_assert(WITHIN(Z_SAFE_HOMING_Y_POINT, Y_MIN_BED, Y_MAX_BED), "Z_SAFE_HOMING_Y_POINT can't be reached by the nozzle.");
+  static_assert(WITHIN(Z_SAFE_HOMING_X_POINT, X_MIN_POS, X_MAX_POS), "Z_SAFE_HOMING_X_POINT can't be reached by the nozzle.");
+  static_assert(WITHIN(Z_SAFE_HOMING_Y_POINT, Y_MIN_POS, Y_MAX_POS), "Z_SAFE_HOMING_Y_POINT can't be reached by the nozzle.");
 #endif
 
 /**