Segfault in ACPI table generation if no PCI devices instantiated

If hyperkit is launched with the -A flag and no PCI devices, it crashes.

In the code below, taken from src/lib/pci_emul.c, bi gets dereferenced even though it's NULL.

static void
pci_bus_write_dsdt(int bus)
{
	struct businfo *bi;

	/*
	 * If there are no devices on this 'bus' then just return.
	 */
	if ((bi = pci_businfo[bus]) == NULL) {
		/*
		 * Bus 0 is special because it decodes the I/O ports used
		 * for PCI config space access even if there are no devices
		 * on it.
		 */
		if (bus != 0)
			return;
	}

	dsdt_fixup(bus, bi->iobase, bi->iolimit, bi->membase32, bi->memlimit32,
		bi->membase64, bi->memlimit64);

	(void) pci_pirq_prt_entry;
	(void) pci_apic_prt_entry;
}

Start in src/lib/pci_emul.c, at pci_bus_write_dsdt, and reproduce hyperkit with the -A flag and no PCI devices. Trace the no-device Bus 0 path and verify that ACPI table generation completes without a segmentation fault when the fix is done.