Repair Large Object bugs demonstrated by Ian Grant's example. inv_write

was inappropriately relying on rel->rd_nblocks to tell if the LO is
empty (apparently a hack to get around a long-dead index bug), causing
misbehavior on a written-but-never-vacuumed LO.  Also, inv_read failed
to cope gracefully with 'holes' (unwritten regions) in the object.