change exists to defined for conf->envelope_sender_header since it always exists since undef is the default
change exists to defined for conf->envelope_sender_header since it always exists since undef is the default

git-svn-id: e9fe9b84e32fc5e57b7d09c40cd78b14ea28ed71@491332 13f79535-47bb-0310-9956-ffa450edef68

apache/spamassassin,apache/spamassassin,apache/spamassassin,apache/spamassassin,apache/spamassassin