fuzz: Avoid the server fuzzer to proceed to the authentication and further

Thanks Phil Turnbull from GitHub

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Norbert Pocs <npocs@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

tests/fuzz/ssh_server_fuzzer.c

@@ -186,6 +186,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
     ssh_set_auth_methods(session, SSH_AUTH_METHOD_NONE);
 
     ssh_callbacks_init(&server_cb);
+    ssh_set_server_callbacks(session, &server_cb);
 
     rc = ssh_bind_accept_fd(sshbind, session, socket_fds[0]);
     assert(rc == SSH_OK);