kvm: x86: fix kvm_apic_has_events to check for NULL pointer

commit ce40cd3f upstream.

Malicious (or egregiously buggy) userspace can trigger it, but it
should never happen in normal operation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Wang Kai <morgan.wang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>