[NTOSKRNL] Don't dereference VACB when allocating its memory area fails

[NTOSKRNL] Don't dereference VACB when allocating its memory area fails

This avoids performing a double-free (even though that's hidden by the
fact we use lookaside allocations for VACB), and it avoids freeing
a memory address at an uninitialized address.
We don't care about references here, the VACB was just allocated, never
linked and we're its only user.

CORE-15413