fs/ntfs3: Fix using uninitialized value n when calling indx_read

This value is checked in indx_read, so it must be initialized
Fixes: 82cae269 ("fs/ntfs3: Add initialization of super block")

Signed-off-by: Yan Lei <chinayanlei2002@163.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>