tnef: Fix clobbered message when an IMAP message has a TNEF attachment

This fixes an issue where, when the mailfetch subsystem fetches more than
one email and one of them has a TNEF attachment; each message body after the
TNEF message will use the TNEF message body.

This patch fixes the issue by clearing the TNEF body in each cycle through
the message fetching and parsing loop.