pgsql: Reducing the assumed alignment of struct varlena means that the

Log Message:
-----------
Reducing the assumed alignment of struct varlena means that the compiler
is also licensed to put a local variable declared that way at an unaligned
address. Which will not work if the variable is then manipulated with
SET_VARSIZE or other macros that assume alignment. So the previous patch
is not an unalloyed good, but on balance I think it's still a win, since
we have very few places that do that sort of thing. Fix the one place in
tuptoaster.c that does it. Per buildfarm results from gypsy_moth
(I'm a bit surprised that only one machine showed a failure).

Modified Files:
--------------
pgsql/src/backend/access/heap:
tuptoaster.c (r1.82 -> r1.83)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/tuptoaster.c?r1=1.82&r2=1.83)