tor_addr_port_lookup() is overly permissive

Like tor_addr_parse() in legacy/trac#23082 (moved), tor_addr_port_lookup() also allows square brackets around IPv4 addresses.

But it's slightly less permissive: it requires a terminating ].

For example, this command line should be rejected, but it is not:

tor ORPort [127.0.0.1]:9001