Check for nil buffers in patty_ax25_sock_destroy()

Check before trying to free() nil buffers in patty_ax25_sock_destroy()

src/sock.c

@@ -198,9 +198,18 @@ void patty_ax25_sock_destroy(patty_ax25_sock *sock) {
         (void)close(sock->fd);
     }
 
+    if (sock->tx_slots) {
         free(sock->tx_slots);
+    }
+
+    if (sock->rx_buf) {
         free(sock->rx_buf);
+    }
+
+    if (sock->tx_buf) {
         free(sock->tx_buf);
+    }
+
     free(sock);
 }