fsck.f2fs: nullify the freed ckpt pointer

Otherwise, we get a double free error by:

In fsck/main.c,

545 out_err:
546         if (sbi->ckpt)
547                 free(sbi->ckpt);

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fsck/mount.c b/fsck/mount.c
index 51d50ec..37a0025 100644
--- a/fsck/mount.c
+++ b/fsck/mount.c
@@ -502,6 +502,7 @@
 
 fail_no_cp:
 	free(sbi->ckpt);
+	sbi->ckpt = NULL;
 	return -EINVAL;
 }