lib-http: Try to reuse memory for host->ips allocation.

If the host already had an IP, most of the time ips_count doesn't change
anymore.

src/lib-http/http-client-host.c

@@ -79,9 +79,9 @@ http_client_host_dns_callback(const struct dns_lookup_result *result,
 		"DNS lookup successful; got %d IPs", result->ips_count);
 
 	i_assert(result->ips_count > 0);
-	i_free(host->ips);
+	host->ips = i_realloc_type(host->ips, struct ip,
+				   host->ips_count, result->ips_count);
 	host->ips_count = result->ips_count;
-	host->ips = i_new(struct ip_addr, host->ips_count);
 	memcpy(host->ips, result->ips, sizeof(*host->ips) * host->ips_count);
 
 	host->ips_timeout = ioloop_timeval;