MINOR: dns: coding style update

No affectation in a if condition.
diff --git a/src/dns.c b/src/dns.c
index 4bc5448..bf5afdf 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -165,7 +165,8 @@
 		query_id = dns_response_get_query_id(buf);
 
 		/* search the query_id in the pending resolution tree */
-		if ((eb = eb32_lookup(&resolvers->query_ids, query_id)) == NULL) {
+		eb = eb32_lookup(&resolvers->query_ids, query_id);
+		if (eb == NULL) {
 			/* unknown query id means an outdated response and can be safely ignored */
 			nameserver->counters.outdated += 1;
 			continue;