Add CVE-2018-1000999 to MailCleaner module

See PR #11148

This adds the new CVE assigned by DWF for this vulnerability.

Note that [CVE-2018-10933](https://www.cvedetails.com/cve/CVE-2018-10933/)
describes a vulnerability in libssh, but this one describes the issue as
it pertains to MailCleaner specifically.

modules/exploits/linux/http/mailcleaner_exec.rb

@@ -26,7 +26,8 @@ class MetasploitModule < Msf::Exploit::Remote
       'References'     =>
         [
           ['URL', 'https://pentest.blog/advisory-mailcleaner-community-edition-remote-code-execution/'],
-          ['CVE', '2018-20323']
+          ['CVE', '2018-20323'],
+          ['CVE', '2018-1000999']
         ],
       'DefaultOptions'  =>
         {