sys/net/nanocoap: fix invalid RST messages

An RST message has no token, so don't reply with a token when sending
RST.

sys/net/application_layer/nanocoap/nanocoap.c

@@ -662,15 +662,15 @@ ssize_t coap_build_reply(coap_pkt_t *pkt, unsigned code,
         return -ENOSPC;
     }
 
-    /* if code is COAP_CODE_EMPTY (zero), assume Reset (RST) type */
-    unsigned type = COAP_TYPE_RST;
-    if (code) {
-        if (coap_get_type(pkt) == COAP_TYPE_CON) {
-            type = COAP_TYPE_ACK;
-        }
-        else {
-            type = COAP_TYPE_NON;
-        }
+    unsigned type = COAP_TYPE_NON;
+    if (!code) {
+        /* if code is COAP_CODE_EMPTY (zero), assume Reset (RST) type.
+         * RST message have no token */
+        type = COAP_TYPE_RST;
+        tkl = 0;
+    }
+    else if (coap_get_type(pkt) == COAP_TYPE_CON) {
+        type = COAP_TYPE_ACK;
     }
 
     uint32_t no_response;