hw/gpio: npcm7xx: fixup out-of-bounds access

The reg isn't validated to be a possible register before
it's dereferenced for one case.  The mmio space registered
for the gpio device is 4KiB but there aren't that many
registers in the struct.

Cc: qemu-stable@nongnu.org
Fixes: 526dbbe0874 ("hw/gpio: Add GPIO model for Nuvoton NPCM7xx")
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250226024603.493148-1-venture@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/gpio/npcm7xx_gpio.c

@@ -220,8 +220,6 @@ static void npcm7xx_gpio_regs_write(void *opaque, hwaddr addr, uint64_t v,
         return;
     }
 
-    diff = s->regs[reg] ^ value;
-
     switch (reg) {
     case NPCM7XX_GPIO_TLOCK1:
     case NPCM7XX_GPIO_TLOCK2:
@@ -242,6 +240,7 @@ static void npcm7xx_gpio_regs_write(void *opaque, hwaddr addr, uint64_t v,
     case NPCM7XX_GPIO_PU:
     case NPCM7XX_GPIO_PD:
     case NPCM7XX_GPIO_IEM:
+        diff = s->regs[reg] ^ value;
         s->regs[reg] = value;
         npcm7xx_gpio_update_pins(s, diff);
         break;