Avoid theoretical NULL pointer dereference during TLS reassemble

This function does not get called with in_data == NULL in practice, but
it seems to be at least partly prepared for that case, so better make it
consistent by handling the NULL value throughout the function.

src/eap_peer/eap_tls_common.c

@@ -361,7 +361,8 @@ static int eap_peer_tls_reassemble_fragment(struct eap_ssl_data *data,
 		eap_peer_tls_reset_input(data);
 		return -1;
 	}
-	wpabuf_put_buf(data->tls_in, in_data);
+	if (in_data)
+		wpabuf_put_buf(data->tls_in, in_data);
 	data->tls_in_left -= in_len;
 
 	if (data->tls_in_left > 0) {