KVM: PPC: Book3S HV: Prevent double-free on HPT resize commit path

resize_hpt_release(), called once the HPT resize of a KVM guest is
completed (successfully or unsuccessfully) frees the state structure for
the resize.  It is currently not safe to call with a NULL pointer.

However, one of the error paths in kvm_vm_ioctl_resize_hpt_commit() can
invoke it with a NULL pointer.  This will occur if userspace improperly
invokes KVM_PPC_RESIZE_HPT_COMMIT without previously calling
KVM_PPC_RESIZE_HPT_PREPARE, or if it calls COMMIT twice without an
intervening PREPARE.

To fix this potential crash bug - and maybe others like it, make it safe
(and a no-op) to call resize_hpt_release() with a NULL resize pointer.

Found by Dan Carpenter with a static checker.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>