hw/timer/pxa2xx_timer: Add assertion to silent static analyzer warning

pxa2xx_timer_tick4() takes an opaque pointer, then calls
pxa2xx_timer_update4(), so the static analyzer can not
verify that the 'n < 8':

  425 static void pxa2xx_timer_tick4(void *opaque)
  426 {
  427     PXA2xxTimer4 *t = (PXA2xxTimer4 *) opaque;
  428     PXA2xxTimerInfo *i = (PXA2xxTimerInfo *) t->tm.info;
  429
  430     pxa2xx_timer_tick(&t->tm);
  433     if (t->control & (1 << 6))
  434         pxa2xx_timer_update4(i, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL), t->tm.num - 4);

  135 static void pxa2xx_timer_update4(void *opaque, uint64_t now_qemu, int n)
  136 {
  137     PXA2xxTimerInfo *s = (PXA2xxTimerInfo *) opaque;
  140     static const int counters[8] = { 0, 0, 0, 0, 4, 4, 6, 6 };
  142
  143     if (s->tm4[n].control & (1 << 7))
  144         counter = n;
  145     else
  146         counter = counters[n];

Add an assert() to give the static analyzer a hint, this fixes a
warning reported by Clang static code analyzer:

    CC      hw/timer/pxa2xx_timer.o
  hw/timer/pxa2xx_timer.c:146:17: warning: Assigned value is garbage or undefined
          counter = counters[n];
                  ^ ~~~~~~~~~~~

Reported-by: Clang Static Analyzer
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422133152.16770-10-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/timer/pxa2xx_timer.c

@@ -140,6 +140,7 @@ static void pxa2xx_timer_update4(void *opaque, uint64_t now_qemu, int n)
     static const int counters[8] = { 0, 0, 0, 0, 4, 4, 6, 6 };
     int counter;
 
+    assert(n < ARRAY_SIZE(counters));
     if (s->tm4[n].control & (1 << 7))
         counter = n;
     else