UBI: fix dereference after kfree

Coverity (CID 1614) spotted new_seb being dereferenced after kfree() in
create_vtbl's write_error path.

Signed-off-by: Florin Malita <fmalita@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>