BUG/MINOR: dns: add test on result getting value from buffer into ring.

This patch adds a missing test in dns_session_io_handler, getting
the query id from the buffer of the ring. An error should never
happen since messages are completely added atomically.

This bug should fix github issue #1133
diff --git a/src/dns.c b/src/dns.c
index 352e88c..bc40b29 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -522,6 +522,13 @@
 
 				/* backup original query id */
 				len = b_getblk(buf, (char *)&original_qid, sizeof(original_qid), ofs + cnt);
+				if (!len) {
+					/* should never happen since messages are atomically
+					 * written into ring
+					 */
+					ret = 0;
+					break;
+				}
 
 				/* generates new query id */
 				new_qid = ++ds->query_counter;