Codechange: Don't save unused NewGRF override mappings. (#9202)

src/saveload/newgrf_sl.cpp

@@ -30,6 +30,7 @@ static const SaveLoad _newgrf_mapping_desc[] = {
 void Save_NewGRFMapping(const OverrideManagerBase &mapping)
 {
 	for (uint i = 0; i < mapping.GetMaxMapping(); i++) {
+		if (!mapping.IsValidID(i)) continue;
 		SlSetArrayIndex(i);
 		SlObject(&mapping.mapping_ID[i], _newgrf_mapping_desc);
 	}