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>

hw/timer/a9gtimer.c

@@ -32,6 +32,7 @@
 #include "qemu/log.h"
 #include "qemu/module.h"
 #include "hw/core/cpu.h"
+#include "sysemu/qtest.h"
 
 #ifndef A9_GTIMER_ERR_DEBUG
 #define A9_GTIMER_ERR_DEBUG 0
@@ -48,6 +49,10 @@
 
 static inline int a9_gtimer_get_current_cpu(A9GTimerState *s)
 {
+    if (qtest_enabled()) {
+        return 0;
+    }
+
     if (current_cpu->cpu_index >= s->num_cpu) {
         hw_error("a9gtimer: num-cpu %d but this cpu is %d!\n",
                  s->num_cpu, current_cpu->cpu_index);