fs/ntfs3: Fix a possible null-pointer dereference in ni_clear()

In a previous commit c1006bd13146, ni->mi.mrec in ni_write_inode()
could be NULL, and thus a NULL check is added for this variable.

However, in the same call stack, ni->mi.mrec can be also dereferenced
in ni_clear():

ntfs_evict_inode(inode)
  ni_write_inode(inode, ...)
    ni = ntfs_i(inode);
    is_rec_inuse(ni->mi.mrec) -> Add a NULL check by previous commit
  ni_clear(ntfs_i(inode))
    is_rec_inuse(ni->mi.mrec) -> No check

Thus, a possible null-pointer dereference may exist in ni_clear().
To fix it, a NULL check is added in this function.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/frecord.c

@@ -102,7 +102,7 @@ void ni_clear(struct ntfs_inode *ni)
 {
 	struct rb_node *node;
 
-	if (!ni->vfs_inode.i_nlink && is_rec_inuse(ni->mi.mrec))
+	if (!ni->vfs_inode.i_nlink && ni->mi.mrec && is_rec_inuse(ni->mi.mrec))
 		ni_delete_all(ni);
 
 	al_destroy(ni);