pgsql: Fix another place that was assuming that a local variable

Log Message:
-----------
Fix another place that was assuming that a local variable declared as
"struct varlena" would be at least word-aligned.  Per buildfarm results
from gypsy_moth.  I did a little bit of trawling for other instances of
this coding pattern, and didn't find any; but if we turn up any more
of them I think we'd better revert the "char [4]" patch and find another
way of making tuptoaster.c alignment-safe.

Modified Files:
--------------
    pgsql/src/backend/storage/large_object:
        inv_api.c (r1.127 -> r1.128)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/large_object/inv_api.c?r1=1.127&r2=1.128)