accel/nvmm: Fix NULL dereference in nvmm_init_vcpu()

accel/nvmm: Fix NULL dereference in nvmm_init_vcpu()

When mechanically moving the @dirty field to AccelCPUState
in commit 79f1926b2d, we neglected cpu->accel is still NULL
when we want to dereference it.

Reported-by: Volker Rümelin <vr_qemu@t-online.de>
Suggested-by: Volker Rümelin <vr_qemu@t-online.de>
Fixes: 79f1926b2d ("accel/nvmm: Use accel-specific per-vcpu @dirty field")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240429091918.27429-3-philmd@linaro.org>