hw/timer/a9gtimer: Handle QTest mode in a9_gtimer_get_current_cpu

hw/timer/a9gtimer: Handle QTest mode in a9_gtimer_get_current_cpu

This commit updates the a9_gtimer_get_current_cpu() function to handle
cases where QTest is enabled. When QTest is used, it returns 0 instead
of dereferencing the current_cpu, which can be NULL. This prevents the
program from crashing during QTest runs.

Reproducer:
cat << EOF | qemu-system-aarch64 -display \
none -machine accel=qtest, -m 512M -machine npcm750-evb -qtest stdio
writel 0xf03fe20c 0x26d7468c
EOF

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240618144009.3137806-1-zheyuma97@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>