xfs: validate quota log items during log recovery

Arkadiusz has seen really strange crashes in xfs_qm_dqcheck that
I can only explain by a log item being too smal to actually fit the
xfs_dqblk_t we're dereferencing all over xfs_qm_dqcheck.  So add
graceful checks for NULL or too small quota items to the log recovery
code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>