Segfault if bind to port 80 is not possible

``` ./acme-client --dir https://pebble:14000/dir --domain example.com --cert=../test/pebble/pebble.minica.pem ```
segfaults after the message "Error: HTTP challenge server bind failed."

This does not occur if --port is correctly set to the port on which pebble is listening (5080 in this case)