Clean up some uses of String::substr

Cases where the end position is either equvalent to the default or past
the end of the string.

main/main.cpp

@@ -4236,7 +4236,7 @@ int Main::start() {
 							Ref<DirAccess> da = DirAccess::open(local_game_path.substr(0, sep));
 							if (da.is_valid()) {
 								local_game_path = da->get_current_dir().path_join(
-										local_game_path.substr(sep + 1, local_game_path.length()));
+										local_game_path.substr(sep + 1));
 							}
 						}
 					}