hw/mips/malta: Silence warning from ubsan

When compiling QEMU with --enable-ubsan there is a undefined behavior
warning when using the malta machine:

 hw/mips/malta.c:1200:32: runtime error: addition of unsigned offset
  to 0x7fb620600000 overflowed to 0x7fb6205fffff
 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior hw/mips/malta.c:1200:32

To fix the issue, check the bios_size whether we really loaded the
firmware before trying to byte-swap the instructions here.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20250728115152.187728-1-thuth@redhat.com>

hw/mips/malta.c

@@ -1191,7 +1191,7 @@ void mips_malta_init(MachineState *machine)
          * In little endian mode the 32bit words in the bios are swapped,
          * a neat trick which allows bi-endian firmware.
          */
-        if (!TARGET_BIG_ENDIAN) {
+        if (!TARGET_BIG_ENDIAN && bios_size > 0) {
             uint32_t *end, *addr;
             const size_t swapsize = MIN(bios_size, 0x3e0000);
             addr = rom_ptr(FLASH_ADDRESS, swapsize);