fs: ntfs3: Fix possible null-pointer dereferences in mi_read()

In a previous commit 2681631c ("fs/ntfs3: Add null pointer check to
attr_load_runs_vcn"), ni can be NULL in attr_load_runs_vcn(), and thus it
should be checked before being used.

However, in the call stack of this commit, mft_ni in mi_read() is
aliased with ni in attr_load_runs_vcn(), and it is also used in
mi_read() at two places:

mi_read()
  rw_lock = &mft_ni->file.run_lock -> No check
  attr_load_runs_vcn(mft_ni, ...)
    ni (namely mft_ni) is checked in the previous commit
  attr_load_runs_vcn(..., &mft_ni->file.run) -> No check

Thus, to avoid possible null-pointer dereferences, the related checks
should be added.

These bugs are reported by a static analysis tool implemented by myself,
and they are found by extending a known bug fixed in the previous commit.
Thus, they could be theoretical bugs.

Signed-off-by: Jia-Ju Bai <baijiaju@buaa.edu.cn>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>