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.