[JFFS2] Check for all-zero node headers

[JFFS2] Check for all-zero node headers

Due to a poor choice of CRC32 seed, a node header which is all zeroes
would pass the CRC32 check. Explicitly check for this case, and treat it
as we do a CRC failure.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>