CLEANUP: dns: Use DISGUISE() on a never-failing ring_attach() call

When a DNS session is created, the call to ring_attach() never fails. The
ring is freshly initialized and there is other watcher on it. Thus, the call
always succeeds.

Instead of catching an error that must never happen, we use the DISGUISE()
macro to make static analyzers happy.
diff --git a/src/dns.c b/src/dns.c
index dcdc163..d360f2c 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -1028,13 +1028,8 @@
 		goto error;
 
 	ring_init(&ds->ring, ds->tx_ring_area, DNS_TCP_MSG_RING_MAX_SIZE);
-	if (!ring_attach(&ds->ring)) {
-		/* Should never happen
-		 * since we are the first attached
-		 * here
-		 */
-		goto error;
-	}
+	/* never fail because it is the first watcher attached to the ring */
+	DISGUISE(ring_attach(&ds->ring));
 
 	if ((ds->task_exp = task_new(tid_bit)) == NULL)
 		goto error;