autoport: Fix nil pointer deref when run without bd82x6x

When autoport is run on a system without supported southbridge
it won't populate the coresponding data structure. By sanitiy
checking after PCI detection autoport can exit cleanly and
provide a sufficient error message.

Error was:
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x4be595]
goroutine 1 [running]:
main.FIXMEEC(0xc42014af80, 0x14, 0xc42014afe0, 0x1a, 0xc4200a914f,
	0x4, 0xc4200a916f, 0xf, 0xc420149e60, 0x28, ...)
/coreboot/util/autoport/ec_fixme.go:14 +0x105

Change-Id: I6b0fcda76d33b0d3a0379c279f492160ce5add84
Signed-off-by: Maximilian Schander <maxschander@googlemail.com>
Reviewed-on: https://review.coreboot.org/22203
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

util/autoport/root.go

@@ -1,6 +1,7 @@
 package main
 
 import "fmt"
+import "os"
 
 var supportedPCIDevices map[uint32]PCIDevice = map[uint32]PCIDevice{}
 var PCIMap map[PCIAddr]PCIDevData = map[PCIAddr]PCIDevData{}
@@ -26,6 +27,10 @@ func ScanRoot(ctx Context) {
 		}
 		dev.Scan(ctx, pciDev)
 	}
+	if SouthBridge == nil {
+		fmt.Println("Could not detect southbridge. Aborting!")
+		os.Exit(1)
+	}
 	dmi := ctx.InfoSource.GetDMI()
 	if !dmi.IsLaptop {
 		NoEC(ctx)