accel/whpx: Fix NULL dereference in whpx_init_vcpu()

accel/whpx: Fix NULL dereference in whpx_init_vcpu()

When mechanically moving the @dirty field to AccelCPUState
in commit 9ad49538c7, we neglected cpu->accel is still NULL
when we want to dereference it.

Fixes: 9ad49538c7 ("accel/whpx: Use accel-specific per-vcpu @dirty field")
Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Suggested-by: Volker Rümelin <vr_qemu@t-online.de>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240429091918.27429-2-philmd@linaro.org>