fs/ntfs3: Check for NULL pointers in ni_try_remove_attr_list

Check for potential NULL pointers.
Print error message if found.
Thread, that leads to this commit:
https://lore.kernel.org/ntfs3/227c13e3-5a22-0cba-41eb-fcaf41940711@paragon-software.com/



Reported-by: Mohammad Rasim <mohammad.rasim96@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>