KVM: nVMX: Set cached_vmcs12 and cached_shadow_vmcs12 NULL after free

Shall help finding use-after-free bugs earlier.
Suggested-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -234,7 +234,9 @@ static void free_nested(struct kvm_vcpu *vcpu)
 		vmx->vmcs01.shadow_vmcs = NULL;
 	}
 	kfree(vmx->nested.cached_vmcs12);
+	vmx->nested.cached_vmcs12 = NULL;
 	kfree(vmx->nested.cached_shadow_vmcs12);
+	vmx->nested.cached_shadow_vmcs12 = NULL;
 	/* Unpin physical memory we referred to in the vmcs02 */
 	if (vmx->nested.apic_access_page) {
 		kvm_release_page_dirty(vmx->nested.apic_access_page);