BUG/MINOR: resolvers: Fix condition to release received ARs if not assigned

BUG/MINOR: resolvers: Fix condition to release received ARs if not assigned

At the end of resolv_validate_dns_response(), if a received additionnal
record is not assigned to an existing server record, it is released. But the
condition to do so is buggy. If "answer_record" (the received AR) is not
assigned, "tmp_record" is not a valid record object. It is just a dummy
record "representing" the head of the record list.

Now, the condition is far cleaner. This patch must be backported as far as
2.2.

(cherry picked from commit 9c246a4b6ce3fa0e70399e0158866d41b8662a7f)
[cf: Changes applied in src/dns.c]
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>