fs/ntfs: Correct possible access violations

fs/ntfs: Correct possible access violations

Correct several memory access violations found during fuzzing.
The issues fixed here could occur if certain specific malformed NTFS
file systems were presented to GRUB. Currently, GRUB does not allow NTFS
file system access when lockdown mode is enforced, so these should be of
minimal impact.

The changes made in this commit generally correct issues where pointers
into data buffers were being calculated using lengths read from the
NTFS file system without sufficient bounds/sanity checking; or
attempting to access elements of a structure to free them, when the
structure pointer is NULL.

Signed-off-by: Andrew Hamilton <adhamilt@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>