vsftpd: use quotes to wrap variable

"test -z ${PAMLIB}" in do_install() may report error if ${PAMLIB}
has space chars. This commit added double quotes to wrap it in
case of errors.

Signed-off-by: Qiang Chen <qiang.chen@windriver.com>
Signed-off-by: Roy Li <rongqing.li@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>