If IORESOURCE_BUS is not provided in Device Tree it will be fabricated in
of_pci_parse_bus_range(), so NULL pointer dereference should not happen
here.

But that's hard to verify, so check for NULL anyway.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Link: https://lore.kernel.org/linux-pci/20240503125705.46055-1-amishin@t-argos.ruSuggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Aleksandr Mishin <amishin@t-argos.ru>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>