Completion of error handling

Would you like to add more error handling for return values from functions like the following?
- [calloc](http://pubs.opengroup.org/onlinepubs/9699919799/functions/calloc.html "Memory allocation") ⇒ [decode_auth_setting](https://github.com/esnet/iperf/blob/88f3ad87f38762081a6d7127890ff5c3a052b06a/src/iperf_auth.c#L264)
- [malloc](http://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html "Memory allocation") ⇒ [add_to_interval_list](https://github.com/esnet/iperf/blob/a68171340fe2ee4fee8585e7a0c8378838d90103/src/iperf_api.c#L2070)