vsftpd: fix pkg_postinst

Fix pkg_postinst to not exit if "$D" is not empty.
Otherwise, postinsts from update-rc.d.bbclass would not run and the
symlinks under /etc/rc?.d/ would not be created.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>