Fix: free pkt_pool memory for keepalive packets (data_len=0)

src/etcp.c

@@ -1010,7 +1010,11 @@ void etcp_ack_recv(struct ETCP_CONN* etcp, uint32_t seq, uint16_t ts, uint16_t d
 // Process incoming decrypted packet
 void etcp_conn_input(struct ETCP_DGRAM* pkt) {
     DEBUG_TRACE(DEBUG_CATEGORY_ETCP, "");
-    if (!pkt || !pkt->data_len) return;
+    if (!pkt) return;
+    if (!pkt->data_len) {
+        memory_pool_free(pkt->link->etcp->instance->pkt_pool, pkt);
+        return;
+    }
 
     etcp_dump_pkt_sections(pkt, pkt->link, 0);