Check if DNS server has been initialized before trying to lookup the server...

Check if DNS server has been initialized before trying to lookup the server address in the cache.  From Librae

libc/netdb/lib_dnsclient.c

@@ -1029,6 +1029,14 @@ int dns_find_answer(FAR const char *hostname, FAR struct sockaddr *addr,
   int next;
   int ndx;
 
+  /* If DNS not initialized, no need to proceed */
+
+  if (!g_dns_initialized)
+    {
+      ndbg("ERROR: DNS not initialized yet\n");
+      return -EAGAIN;
+    }
+
   /* Get exclusive access to the DNS cache */
 
   dns_semtake();