ntfs3: avoid another -Wmaybe-uninitialized warning

ntfs3: avoid another -Wmaybe-uninitialized warning

The ntfs3 specific -Wmaybe-uninitialized flag found one more false-postive,
this time with gcc-10 on s390:

fs/ntfs3/frecord.c: In function 'ni_expand_list':
fs/ntfs3/frecord.c:1370:16: error: 'ins_attr' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Add an explicit NULL pointer check before using the pointer, and
initialize it to NULL.

Fixes: 48d9b57b169f ("fs/ntfs3: add a subset of W=1 warnings for stricter checks")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>