hw/intc/openpic: Avoid taking address of out-of-bounds array index

hw/intc/openpic: Avoid taking address of out-of-bounds array index

The clang sanitizer complains about the code in the EOI handling
of openpic_cpu_write_internal():

UBSAN_OPTIONS=halt_on_error=1:abort_on_error=1 ./build/clang/qemu-system-ppc -M mac99,graphics=off -display none -kernel day15/invaders.elf
../../hw/intc/openpic.c:1034:16: runtime error: index -1 out of bounds for type 'IRQSource[264]' (aka 'struct IRQSource[264]')
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../hw/intc/openpic.c:1034:16 in

This is because we do
  src = &opp->src[n_IRQ];
when n_IRQ may be -1.  This is in practice harmless because if n_IRQ
is -1 then we don't do anything with the src pointer, but it is
undefined behaviour. (This has been present since this device
was first added to QEMU.)

Rearrange the code so we only do the array index when n_IRQ is not -1.

Cc: qemu-stable@nongnu.org
Fixes: e9df014c0b ("Implement embedded IRQ controller for PowerPC 6xx/740 & 75")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-id: 20241105180205.3074071-1-peter.maydell@linaro.org
(cherry picked from commit 3bf7dcd47a3da0e86a9347ce5b2b5d5a1dcb5857)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>