BUG/MINOR: dns: fix ring attach control on dns_session_new

Ths patch adds a control on ring_attach which can not currently fail
since we are the first to try to attach.

This should fix issue #1126
diff --git a/src/dns.c b/src/dns.c
index 9e44dd6..58c68d7 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -1033,7 +1033,13 @@
 		goto error;
 
 	ring_init(&ds->ring, ds->tx_ring_area, DNS_TCP_MSG_RING_MAX_SIZE);
-	ring_attach(&ds->ring);
+	if (!ring_attach(&ds->ring)) {
+		/* Should never happen
+		 * since we are the first attached
+		 * here
+		 */
+		goto error;
+	}
 
 	if ((ds->task_exp = task_new(tid_bit)) == NULL)
 		goto error;