Fix error message when a hostaddr cannot be parsed.

We were incorrectly passing hostname, not hostaddr, in the error message,
and because of that, you got:

$ psql 'hostaddr=foo'
psql: could not parse network address "(null)": Name or service not known

Backpatch to v10, where this was broken (by commit 7b02ba62).

Report and fix by Robert Haas.

Discussion: https://www.postgresql.org/message-id/CA+TgmoapFQA30NomGKEaZCu3iN7mF7fux8fbbk9SouVOT2JP7w@mail.gmail.com