P2P: Be more careful with wpa_config_update_psk() call
Commit bb4d4deb4bbb7b094faa16fa41f1f49b062cbf22 introduced a code path
that could potentially end up calling wpa_config_update_psk() with NULL
passphrase. While that may not happen in practice, it is better to make
sure it doesn't happen since that function will dereference the
passphrase pointer unconditionally.
Signed-hostap: Jouni Malinen <j@w1.fi>