KVM: x86: Fix apic dangling pointer in vcpu

The function kvm_create_lapic() attempts to allocate the apic structure
and sets a pointer to it in the virtual processor structure. However, if
get_zeroed_page() failed, the function frees the apic chunk, but forgets
to set the pointer in the vcpu to NULL. It's not a security issue since
there isn't a use of that pointer if kvm_create_lapic() returns error,
but it's more accurate that way.

Signed-off-by: Saar Amar <saaramar@microsoft.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>