acpi: handle machines with no MCFG

Syzkaller died during early boot after ACPI init and before/during PCI.
It's likely the VM it runs in doesn't have an MCFG, and we weren't
handling that case when PCI queried a device's MMIO config space.

Reported-by: syzbot+3feb100d5398d8b5d728@syzkaller.appspotmail.com
Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

kern/drivers/dev/acpi.c

diff --git a/kern/drivers/dev/acpi.c b/kern/drivers/dev/acpi.c
index aa9f971..91e4d09 100644
--- a/kern/drivers/dev/acpi.c
+++ b/kern/drivers/dev/acpi.c
@@ -1409,7 +1409,11 @@
 
 physaddr_t acpi_pci_get_mmio_cfg_addr(int segment, int bus, int dev, int func)
 {
-	struct acpi_mcfg_data *tbl = mcfgtbl->tbl;
+	struct acpi_mcfg_data *tbl;
+
+	if (!mcfgtbl)
+		return 0;
+	tbl = mcfgtbl->tbl;
 
 	for (int i = 0; i < tbl->nr_entries; i++) {
 		if (segment == tbl->entries[i].segment &&