Fix crash when no callback is defined

libssh/keyfiles.c

@@ -677,7 +677,7 @@ ssh_private_key privatekey_from_file(ssh_session session, const char *filename,
   switch (type) {
     case TYPE_DSS:
       if (passphrase == NULL) {
-        if (session->callbacks->auth_function) {
+        if (session->callbacks && session->callbacks->auth_function) {
           auth_cb = session->callbacks->auth_function;
           auth_ud = session->callbacks->userdata;