proftpd: fix integer overflow CVE-2013-4359

Integrate a patch from proftpd upstream to fix an integer overflow bug
described in the CVE-2013-4359, which allows remote attachers to cause
a denial of service (memory consumption) attack.

Refer: http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-4359



Signed-off-by: Shan Hai <shan.hai@windriver.com>

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>