spapr_pci: Fix potential NULL pointer dereference in spapr_dt_pci_bus()

spapr_pci: Fix potential NULL pointer dereference in spapr_dt_pci_bus()

Commit 14e714900f6 refactored the call to spapr_dt_drc(),
introducing a potential NULL pointer dereference while
accessing bus->parent_dev.
A trivial audit show 'bus' is not null in the two places
the static function spapr_dt_drc() is called.

Since the 'bus' parameter is not NULL in both callers, remove
remove the test on if (bus), and add an assert() to silent
static analyzers.

This fixes:

  /hw/ppc/spapr_pci.c: 1367 in spapr_dt_pci_bus()
  >>>     CID 1401933:  Null pointer dereferences  (FORWARD_NULL)
  >>>     Dereferencing null pointer "bus".
  1367         ret = spapr_dt_drc(fdt, offset, OBJECT(bus->parent_dev),
  1368                            SPAPR_DR_CONNECTOR_TYPE_PCI);

Fixes: 14e714900f6
Reported-by: Coverity (CID 1401933)
Suggested-by: Greg Kurz <groug@kaod.org>
Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190613213406.22053-1-philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>