[An extra null pointer check is not needed in functions](https://stackoverflow.com/questions/18775608/free-a-null-pointer-anyway-or-check-first "Free a null pointer anyway or check first?") like the following. - [cd_set_catalog](https://github.com/lipnitsk/libcue/blob/7176a1faccecbfe2d4cca2f776177439ca49cad2/cd.c#L157) - [track_set_filename](https://github.com/lipnitsk/libcue/blob/7176a1faccecbfe2d4cca2f776177439ca49cad2/cd.c#L227)
