Check the return value of tor_inet_ntop() and tor_inet_ntoa()

The following functions don't check the return value of tor_inet_ntop() or tor_inet_ntoa():

IPv6, could be serious:

• evdns_callback(), multiple times

IPv4 only, unlikely to be a serious bug:

• tor_dup_ip()
• fmt_addr32()
• evdns_wildcard_check_callback()

These functions should log a bug log using BUG() or tor_assert_nonfatal(), and return an error. (Or for the formatting functions, a sensible placeholder string.)

We will also need to make their callers check for the error.