Filter empty block from uninc change before incorporation.

Filter empty block from uninc change before incorporation.

It is possible (though unusual) for an uninc chain to have
two block with the same fileaddr, one that is empty and being ignored,
and one that is newly split off and needs to be incorporated.
We need to detect this possibility after sorting and discard the
empty block so it doesn't confuse further incorporation.

Signed-off-by: NeilBrown <neilb@suse.de>