Repair erroneous use of hashvarlena() for MACADDR, which is not a

Repair erroneous use of hashvarlena() for MACADDR, which is not a
varlena type.  (I did not force initdb, but you won't see the fix
unless you do one.)  Also, make sure all index support operators and
functions are careful not to leak memory for toasted inputs; I had
missed some hash and rtree support ops on this point before.