package/pure-ftpd: fix CVE-2021-40524

In Pure-FTPd 1.0.49, an incorrect max_filesize quota mechanism in the
server allows attackers to upload files of unbounded size, which may
lead to denial of service or a server hang. This occurs because a
certain greater-than-zero test does not anticipate an initial -1 value.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>