f2fs: fix to do sanity check in is_alive()

In fuzzed image, SSA table may indicate that a data block belongs to
invalid node, which node ID is out-of-range (0, 1, 2 or max_nid), in
order to avoid migrating inconsistent data in such corrupted image,
let's do sanity check anyway before data block migration.

Cc: stable@vger.kernel.org
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>