V2Writeable.pm: Ensure that a found message number is in the msgmap
The lookup to see if a num has already been assigned to a message
happens in a temporary copy of message map. It is possible that the
number has been removed from the current message map. The
unindex/reindex after a history rewrite triggered by a purge should be
one such case. Therefore add the number to the msgmap in case it is
not currently present.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>