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

In the Linux kernel, the following vulnerability has been resolved:

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.