fsck.f2fs: nullify raw_super pointer

Otherwise it will cause double free.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fsck/mount.c b/fsck/mount.c
index c4dfb0a..4c807f9 100644
--- a/fsck/mount.c
+++ b/fsck/mount.c
@@ -339,6 +339,7 @@
 	}
 
 	free(sbi->raw_super);
+	sbi->raw_super = NULL;
 	MSG(0, "\tCan't find a valid F2FS superblock at 0x%x\n", block);
 
 	return -EINVAL;