UBI: fix double free on error path

If we fail in 'ubi_eba_init_scan()', we free
'ubi->volumes[i]->eba_tbl' in there, but also later free it
in 'free_internal_volumes()'. Fix this by assigning NULL
to 'ubi->volumes[i]->eba_tbl' after it is freed.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>