LP 1786534: Don't merge a user with itself.

LP 1786534: Don't merge a user with itself.

It's possible to merge a user with itself using the actor.usr_merge
database function.  This does not produce a useful result.  One ends
up with a user that is flagged deleted, has a null card field, and is
likely to have their addresses and other cards deleted from the
database.

A simple check at the top of the database function to bail if the
src_usr and dest_usr are the same prevents this from happening.  That
is the change that this commit makes.

Signed-off-by: Jason Stephenson <jason@sigio.com>