fs/ntfs3: Drop redundant NULL check

Static analysis shows that pointer "mi" cannot be NULL, since it is
pre-initialized above. A potential failure when mi equals NULL is
processed.

Remove the extra NULL check. It is meaningless and harms the readability
of the code, since before that the pointer is unconditionally
dereferenced.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Andrey Vatoropin <a.vatoropin@crpt.ru>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>