fetchmail workaround for a bug in Zimbra

fetchmail workaround for a bug in Zimbra

Zimbra occasionally returns this response:

fetchmail: IMAP> A0007 FETCH 1 RFC822.HEADER
fetchmail: IMAP< * 1 FETCH ()
fetchmail: IMAP< A0007 OK FETCH completed

It happens when there is a corrupted message without a header in the
database. (I don't know how this message could be created, I just see it
there).

When fetchmail encounters such resonse, it gives up and disconnects.

This patch changes it so that PS_TRANSIENT is returned in this
case and fetchmail continues to fetch following messages correctly.