viacam: Don't explode if pci_find_bus() returns NULL

In the unlikely case that pci_find_bus() should return NULL
viacam_serial_is_enabled() is going to dereference a NULL pointer and
blow up. Better safe than sorry, so be defensive and check the
pointer.
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/media/video/via-camera.c

@@ -1332,6 +1332,8 @@ static __devinit bool viacam_serial_is_enabled(void)
 	struct pci_bus *pbus = pci_find_bus(0, 0);
 	u8 cbyte;
 
+	if (!pbus)
+		return false;
 	pci_bus_read_config_byte(pbus, VIACAM_SERIAL_DEVFN,
 			VIACAM_SERIAL_CREG, &cbyte);
 	if ((cbyte & VIACAM_SERIAL_BIT) == 0)