hw/adc: Fix out-of-bounds write in Aspeed ADC model

The 'regs' array has ASPEED_ADC_NR_REGS (52) elements, while the
memory region covers offsets 0x00-0xFC. The aspeed_adc_engine_write()
function has an out-of-bounds write vulnerability when accessing
unimplemented registers.

Fix this by using 'return' instead of 'break' in the default case,
which prevents execution from reaching the s->regs[reg] assignment for
unimplemented registers.

Reported-by: Elhrj Saad <saadelhrj@gmail.com>
Fixes: 5857974d5d ("hw/adc: Add basic Aspeed ADC model")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20260126141820.719492-1-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/adc/aspeed_adc.c

@@ -228,7 +228,8 @@ static void aspeed_adc_engine_write(void *opaque, hwaddr addr, uint64_t value,
         qemu_log_mask(LOG_UNIMP, "%s: engine[%u]: "
                       "0x%" HWADDR_PRIx " 0x%" PRIx64 "\n",
                       __func__, s->engine_id, addr, value);
-        break;
+        /* Do not update the regs[] array */
+        return;
     }
 
     s->regs[reg] = value;