sim_irq: guard NULL pool in avr_irq_register_notify during reset

[dmcgougan]

Interrupt-vector IRQs are created with pool == NULL. avr_watchdog_reset() re-registers its notify hook from within avr_reset (while avr->resetting is set), so avr_irq_register_notify() dereferences irq->pool->avr->resetting on an IRQ whose pool is NULL and segfaults on any watchdog reset.

Guard the NULL pool so the reset path is safe; the resetting-time value/flag reset simply does not apply to pool-less IRQs.

[gatk555]

Please post code and instructions to reproduce the crash.

[gatk555]

Those IRQs are linked to the AVR pool like all the rest. This looks like raw LLM slop.

[dmcgougan]

You are right. Normal interrupt vectors belong to avr->irq_pool. The ATmega128 has no watchdog interrupt, so simavr declares it with _VECTOR(0), but avr_watchdog_reset() still registers an IRQ callback unconditionally. The generic null check is therefore wrong; the fix I should have submitted belongs in the simavr watchdog code:

if (p->watchdog.vector)
avr_irq_register_notify(p->watchdog.irq, avr_watchdog_irq_notify, p);

To reproduce, run ATmega128 code that triggers watchdog reset using this AVR sequence:

wdt_enable(WDTO_15MS);
for (;;) { }

It crashes simavr. Physical ATmega128 hardware resets normally.

Submitted PR #587 to replace this one.