- fix pci/probe.c possible NULL

Hi,

In drivers/pci/probe.c::pci_scan_bridge() the call for pci_alloc_child_bus()
can return NULL, but it is not handled by the function (detected by Coverity's checker).

The patch bellow fix that returning `max' if we got the NULL, but
I do not know if it is right. I guess it is, because in that case the function will act in the same way as with `pass != 0'.