Allocation error handling

Would you like to add more error handling for return values from functions like the following?
- [malloc](http://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html "Memory allocation") ⇒ [interactive_next](https://github.com/emersion/mrsh/blob/eaf18a7e54797ec42929f370acafb8ab2a549e4e/frontend/readline.c#L30)
- [strdup](http://pubs.opengroup.org/onlinepubs/9699919799/functions/strdup.html "Duplicate a string.") ⇒ [mrsh_hashtable_set](https://github.com/emersion/mrsh/blob/442073258468d51704f72b33ba32d4b7be17227f/hashtable.c#L30)