UBI: fix backward compatibility

Commit 32bc4820287a1a03982979515949e8ea56eac641 did not fully fix
the backward compatibility issues. We still fail to properly handle
situations when the first PEB contains non-zero image sequence
number, but one of the following PEBs contains zero image sequence
number. For example, this may happen if we mount a new image with
an old kernel, and then try to mount it in the new kernel.

This patch should fix the issue.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>