Avoid access of a Xapian iterator's object when there's nothing there.

Avoid access of a Xapian iterator's object when there's nothing there.

This eliminates a crash when a message (either corrupted or a non-mail
file that wasn't properly detected as not being mail) has no In-Reply-To
header, (and so few terms that trying to skip to the prefix of the
In-Reply-To terms actually brings us to the end of the termlist).